The contrast between the colors for the background and the text in the Tooltip of the Chart series is very low (dark background and black text). https://www.screencast.com/t/fhx6937j

The problem is most obvious in the Outlook, Simple, Web20 and Sunset skins. 

Workaround would be  setting the Color property for the ToolTipAppearance to White

For the time being you can set the types through the kendoWidget:
		<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
			<script>
				function pageLoad() {
					var chart = $find('<%=RadHtmlChart1.ClientID%>');
					chart._chartObject.options.series[0].type = "verticalLine";
					chart.repaint();
				}
			</script>
		</telerik:RadCodeBlock>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="23000" />
							<telerik:CategorySeriesItem Y="10000" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>

For the time being the following workaround can be used:

JavaScript:

    <script>
        function pageLoad() {
            var chart = $find("<%=RadHtmlChart1.ClientID%>");
            chart._chartObject.options.series[0].labels.background = "yellow";
            chart._chartObject.options.series[0].labels.border = {
                width: 2,
                dashType: "dashDot",
                color: "green"
            };
            chart.repaint();
        }
    </script>

ASPX:

        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries>
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="10" />
                            <telerik:CategorySeriesItem Y="30" />
                            <telerik:CategorySeriesItem Y="20" />
                            <telerik:CategorySeriesItem Y="25" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>

For the time being the property can be set through the underlying Kendo Chart widget. For example:

		<script>
			function OnLoad(chart) {
				var widget = chart.get_kendoWidget();
				//also applies for the minor grid lines - replace majorGridLines with minorGridLines
				//Numeric series
				widget.options.xAxis.majorGridLines.step = 5;
				widget.options.yAxis.majorGridLines.step = 5;
				//Category series
				//widget.options.categoryAxis.majorGridLines.step = 5;
				//widget.options.valueAxis.majorGridLines.step = 5;
				widget.redraw();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="BubbleChart" Width="500" Height="400">
			<ClientEvents OnLoad="OnLoad" />
			<ChartTitle Text="Market Share Study">
			</ChartTitle>
			<PlotArea>
				<Appearance>
					<FillStyle BackgroundColor="White"></FillStyle>
				</Appearance>
				<XAxis MinValue="0" MaxValue="100" Step="10">
					<MinorGridLines Visible="false" />
				</XAxis>
				<YAxis MinValue="0" MaxValue="100" Step="10">
					<MinorGridLines Visible="false" />
				</YAxis>
				<Series>
					<telerik:BubbleSeries>
						<Appearance FillStyle-BackgroundColor="#6ab2c9">
						</Appearance>
						<TooltipsAppearance DataFormatString="Percentage of Market Share: {2}%<br /> Number of products: {0}<br /> Sales: ${1}" />
						<SeriesItems>
							<telerik:BubbleSeriesItem Size="3" X="5" Y="55" />
							<telerik:BubbleSeriesItem Size="12" X="14" Y="80" />
							<telerik:BubbleSeriesItem Size="33" X="20" Y="60" />
							<telerik:BubbleSeriesItem Size="10" X="18" Y="24" />
							<telerik:BubbleSeriesItem Size="42" X="22" Y="32" />
						</SeriesItems>
					</telerik:BubbleSeries>
				</Series>
			</PlotArea>
			<Legend>
				<Appearance Position="Right"></Appearance>
			</Legend>
		</telerik:RadHtmlChart>

For the time being the function can be set through the kendoWidget:

<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Transitions="true">
    <PlotArea>
        <Series>
            <telerik:ColumnSeries Name="Product 1">
                <TooltipsAppearance BackgroundColor="Orange" DataFormatString="{0} sales" />
                <SeriesItems>
                    <telerik:CategorySeriesItem Y="15000" />
                    <telerik:CategorySeriesItem Y="23000" />
                    <telerik:CategorySeriesItem Y="10000" />
                    <telerik:CategorySeriesItem Y="16000" />
                </SeriesItems>
            </telerik:ColumnSeries>
        </Series>
        <XAxis AxisCrossingValue="0" Color="Black" MajorTickType="Outside" MinorTickType="Outside"
            Reversed="false">
            <Items>
                <telerik:AxisItem LabelText="1" />
                <telerik:AxisItem LabelText="2" />
                <telerik:AxisItem LabelText="3" />
                <telerik:AxisItem LabelText="4" />
            </Items>
            <LabelsAppearance DataFormatString="Q{0}" RotationAngle="0" />
            <MajorGridLines Color="#EFEFEF" Width="1" />
            <MinorGridLines Color="#F7F7F7" Width="1" />
            <TitleAppearance Position="Center" RotationAngle="0" Text="Quarters" />
        </XAxis>
        <YAxis AxisCrossingValue="0" Color="Black" MajorTickSize="1" MajorTickType="Outside"
            MaxValue="50000" MinorTickSize="1" MinorTickType="Outside" MinValue="0" Reversed="false"
            Step="10000">
            <LabelsAppearance DataFormatString="{0} sales" RotationAngle="0" />
            <TitleAppearance Position="Center" RotationAngle="0" Text="Sales" />
        </YAxis>
    </PlotArea>
    <ChartTitle Text="Product Sales for 2011">
    </ChartTitle>
    <Legend>
        <Appearance Position="Bottom" />
    </Legend>
</telerik:RadHtmlChart>
<script>
    function pageLoad() {
        $ = $telerik.$;
        var kendoWidget = $find("<%=ColumnChart1.ClientID%>").get_kendoWidget();
        kendoWidget.options.series[0].labels.position = function (point) {
            if (point.value < 20000) {
                console.log(1);
                return "insideEnd";
            } else {
                console.log(2);
                return "outsideEnd";
            }
        }
        kendoWidget.redraw();
    }
</script>

A good improvement in the RadHtmlChart would be to add MaxSize and MinSize properties for the series, which would allow the control the scaling of the bubble chart item. The property can be set through the chartObject:

        <script>
            function pageLoad() {
                var chart = $find("<%=BubbleChart.ClientID%>");
                chart.get_kendoWidget().options.series[0].minSize = 1;
                chart.get_kendoWidget().options.series[0].maxSize = 10;
                chart.get_kendoWidget().redraw();
            }
        </script>
        ASPX:
        <telerik:RadHtmlChart runat="server" ID="BubbleChart" Width="500" Height="400">
            <PlotArea>
                <Series>
                    <telerik:BubbleSeries>
                        <Appearance FillStyle-BackgroundColor="#6ab2c9">
                        </Appearance>
                        <SeriesItems>
                            <telerik:BubbleSeriesItem Size="1" X="5" Y="5500" />
                            <telerik:BubbleSeriesItem Size="5" X="14" Y="12200" />
                            <telerik:BubbleSeriesItem Size="15" X="20" Y="39000" />
                        </SeriesItems>
                    </telerik:BubbleSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>


More detailed information regarding these properties can be found in the API reference of the Kendo UI Chart here:
http://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart#configuration-series.maxSize

Hi , help to change the RadChart To RadHtmlChart for line chart series , below is the existing code used RadChart . i need to update to RadHtmlChart

hartAnalysis_BLL objBLL = new ChartAnalysis_BLL();
DataTable dt = objBLL.GetMonthlySSMTotalQuantityImported(arrParameters);

// Create new chart
RadChart2.ChartTitle.TextBlock.Appearance.TextProperties.Font = new System.Drawing.Font(RadChart2.ChartTitle.TextBlock.Appearance.TextProperties.Font.FontFamily, 11, System.Drawing.FontStyle.Bold);
RadChart2.ChartTitle.TextBlock.Text = "Extended Monthly SSM Total Quantity Imported \n(" + ssm[0] + ")";
RadChart2.Width = AppConstants.CONST_CHART_WIDTH;
RadChart2.ChartTitle.Visible = true;
RadChart2.Chart.DefaultType = ChartSeriesType.Line;
RadChart2.PlotArea.Appearance.Dimensions.Margins.Left = Telerik.Charting.Styles.Unit.Percentage(10);
RadChart2.PlotArea.Appearance.Dimensions.Margins.Right = Telerik.Charting.Styles.Unit.Percentage(15);
RadChart2.PlotArea.Appearance.Dimensions.Margins.Bottom = Telerik.Charting.Styles.Unit.Percentage(30);

RadChart2.DataManager.ValuesXColumn = "Date";
RadChart2.DataManager.ValuesYColumns = new string[1] { "SSM Quantity" };
RadChart2.IntelligentLabelsEnabled = true;
RadChart2.DataSource = dt;
RadChart2.DataBind();
RadChart2.Skin = AppConstants.CONST_TELERIK_CHARTSKIN;

RadChart2.PlotArea.XAxis.Clear();
RadChart2.PlotArea.XAxis.IsZeroBased = false;
RadChart2.PlotArea.XAxis.AutoScale = false;
RadChart2.PlotArea.XAxis.Appearance.ValueFormat = ChartValueFormat.ShortDate;
RadChart2.PlotArea.XAxis.Appearance.CustomFormat = "MMM yy";
RadChart2.PlotArea.XAxis.AxisLabel.Visible = true;
RadChart2.PlotArea.XAxis.AxisLabel.TextBlock.Text = "Month \n(" + AppFunctions.GetDisclaimer() + ")";
RadChart2.PlotArea.XAxis.AxisLabel.TextBlock.Appearance.TextProperties.Color = System.Drawing.Color.Black;
RadChart2.PlotArea.XAxis.Appearance.LabelAppearance.RotationAngle = -45;

RadChart2.PlotArea.YAxis.AxisLabel.Visible = true;
RadChart2.PlotArea.YAxis.AxisLabel.TextBlock.Text = "Total Quantity of SSM Imported (" + dt.Rows[0]["UOM"] + ")";
RadChart2.PlotArea.YAxis.AxisLabel.TextBlock.Appearance.TextProperties.Color = System.Drawing.Color.Black;

RadChart2.Series.GetSeries(0).Appearance.PointMark.Dimensions.Width = 1;
RadChart2.Series.GetSeries(0).Appearance.PointMark.Dimensions.Height = 1;
RadChart2.Series.GetSeries(0).Appearance.PointMark.FillStyle.MainColor = System.Drawing.Color.Black;
RadChart2.Series.GetSeries(0).Appearance.PointMark.FillStyle.FillType = FillType.Solid;
RadChart2.Series.GetSeries(0).Appearance.PointMark.Visible = true;
RadChart2.Series.GetSeries(0).Appearance.LineSeriesAppearance.Color = System.Drawing.Color.Orange;
RadChart2.Series.GetSeries(0).Appearance.LineSeriesAppearance.Width = 3;

int intYear = dt_SelDateFrom.Year;
int intMonth = dt_SelDateFrom.Month;
DateTime start = new DateTime(intYear, intMonth, 1);
TimeSpan ts = dt_SelDateTo - dt_SelDateFrom;
int intMthDiff = Convert.ToInt32(Math.Ceiling((ts.TotalDays / 30)));

for (int i = 0; i < intMthDiff; i++)
{
    ChartAxisItem item = new ChartAxisItem();
    item.Value = (decimal)start.AddMonths(i).ToOADate();
    RadChart2.PlotArea.XAxis.AddItem(item);
}

DataTable dt_avg = objBLL.GetSSMTotalImportedLastYearAverage(arrParameters);

// Define chart series
ChartSeries avg_series = new ChartSeries();
avg_series.Appearance.LabelAppearance.Visible = false;
avg_series.Name = "Last Year Average";
avg_series.Type = ChartSeriesType.Line;
avg_series.Appearance.PointMark.Dimensions.Width = 1;
avg_series.Appearance.PointMark.Dimensions.Height = 1;
avg_series.Appearance.PointMark.FillStyle.MainColor = System.Drawing.Color.Black;
avg_series.Appearance.PointMark.FillStyle.FillType = FillType.Solid;
avg_series.Appearance.PointMark.Visible = true;
avg_series.Appearance.LineSeriesAppearance.Color = System.Drawing.Color.Red;

foreach (DataRow row in dt_avg.Rows)
{
    foreach (DataColumn col in dt_avg.Columns)
    {
        Decimal dec_data = Convert.ToDecimal(row[col]);
        Double dbl_data = Convert.ToInt32(dec_data);
        avg_series.AddItem(dbl_data);
    }
}

RadChart2.AddChartSeries(avg_series);

ArrayList arrParameterBaseline = new ArrayList();
arrParameterBaseline.Add(dt_SelBaselineFrom);
arrParameterBaseline.Add(dt_SelBaselineTo);
arrParameterBaseline.Add(dt_SelSSMSeqID);
SqlDataReader dr_baseline = objBLL.GetSSMTotalImportedBaseline(arrParameterBaseline);

ChartSeries baseline_series = new ChartSeries();
baseline_series.Appearance.LabelAppearance.Visible = false;
baseline_series.Name = "Baseline Average";
baseline_series.Type = ChartSeriesType.Line;
baseline_series.Appearance.PointMark.Dimensions.Width = 1;
baseline_series.Appearance.PointMark.Dimensions.Height = 1;
baseline_series.Appearance.PointMark.FillStyle.MainColor = System.Drawing.Color.Black;
baseline_series.Appearance.PointMark.FillStyle.FillType = FillType.Solid;
baseline_series.Appearance.PointMark.Visible = true;
baseline_series.Appearance.LineSeriesAppearance.Color = System.Drawing.Color.Chartreuse;

Double dbl_baseline = 0;

if (dr_baseline == null)
{
    dbl_baseline = 0;
}
else
{
    try
    {
        while (dr_baseline.Read())
        {
            if (dr_baseline["TotalImportedBaseline"] == DBNull.Value)
                dbl_baseline = 0;
            else
            {
                Decimal dec_baseline = dr_baseline.GetDecimal(dr_baseline.GetOrdinal("TotalImportedBaseline"));
                dbl_baseline = Convert.ToDouble(dec_baseline);
            }
        }
    }
    catch (Exception ex)
    {
        AppFunctions.SetMessageError(this.Page, AppConstants.CONST_ERROR_LOAD_CHART);
        AppFunctions.ErrorLog(ex);
    }
    finally
    {
        if (dr_baseline != null)
        {
            dr_baseline.Close();
            dr_baseline.Dispose();
        }
    }
}

for (int i = 0; i < intMthDiff; i++)
{
    baseline_series.AddItem(dbl_baseline);
}

RadChart2.AddChartSeries(baseline_series);

Panel1.Controls.Add(RadChart2);

}
catch (Exception ex)
{
    AppFunctions.SetMessageError(this.Page, AppConstants.CONST_ERROR_LOAD_CHART);
    AppFunctions.ErrorLog(ex);
}

Test Environment:

For the time being the function can be set through the kendoWidget:
ASPX:
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			function pageLoad() {
				var chart = $find("<%=RadHtmlChart1.ClientID%>");
				chart._chartObject.options.series[0].markers.type = function (point) {
					if (point.value > 2) {
						return "triangle";
					}
					else {
						return "circle";
					}
				};
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="1" />
							<telerik:CategorySeriesItem Y="2" />
							<telerik:CategorySeriesItem Y="3" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>

The current implementation of RadHtmlChart explicitly specifies font-family and font-size settings on the server-side, which are then serialized and used by the Kendo widget, so the skin-specific settings are ignored.

For example, the Material skin should define a "Roboto" font-family, but this is overriden by "Arial" in the final serialization, so the skin configuration does not have any effect.

The default values of the server-side font settings can be removed in order to allow such customization and can be defined in the skins file instead.

Under the standard definition of data format strings, range scaling can be applied (i.e. divide by 1000) using , prefixing the implied or explicit decimal place.
e.g. DataFormatString="{0:#,#,}" will display 3000000 as 3,000

This works on RadPivotGrid correctly, but RadHTMLChart does not work either on data labels, axis labels, or pretty much anywhere else. Apparently a problem with kendo.format(), which does not do this as per documentation for DataFormatString

N.B. This is commonly used in finance presentations to globally present in $thousands.

When you configure RadHtmlChart's YAxis with NarrowRange set to true, the setting is not serialized and as a result, not applied.
You can avoid it by setting the narrowRange on the client-side:

		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="250px" Height="300px">
			<ClientEvents OnLoad="chartLoad" />
			...
		</telerik:RadHtmlChart>
		<script>
			function chartLoad(){
				var chart = $telerik.$("#RadHtmlChart1").data("kendoChart");
				chart.options.valueAxis.narrowRange = true;
			}
		</script>

 Workaround use padding instead of margin:
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server"  Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="-4000" />
							<telerik:CategorySeriesItem Y="10000" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<LabelsAppearance RotationAngle="45">
						<TextStyle Padding="135 0 0 0" />
					</LabelsAppearance>
					<Items>
						<telerik:AxisItem LabelText="item1" />
						<telerik:AxisItem LabelText="item2" />
						<telerik:AxisItem LabelText="item3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>

The functionality is similar to the one in Kendo UI Chart (http://demos.telerik.com/kendo-ui/dataviz/line-charts/notes.html) :
http://dojo.telerik.com/OZuNU

For the time being you can try to utilize the kendo.drawing API in order to place the text on a particular position.  This forum may be helpful on the matter - http://www.telerik.com/forums/custom-drawing#sOkmYWbfrUW404Rr13eLDQ

Currently, the HighlightAppearance of the Series does not have a property that controls the InactiveOpacity:

• https://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart/configuration/series.highlight#serieshighlightinactiveopacity 

As a temporary workaround, this can be set through the options in the OnKendoWidgetInitializing event:

<script>
    function OnKendoWidgetInitializing(sender, args) {
        var series = args.series[0]
        series.highlight = series.highlight || {};
        series.highlight.inactiveOpacity = 0.2
    }
</script>
<telerik:RadHtmlChart ID="RadHtmlChart5" runat="server" BorderColor="Fuchsia" BorderStyle="Solid" BorderWidth="1px">
    <ClientEvents OnKendoWidgetInitializing="OnKendoWidgetInitializing" />
    <PlotArea>

        <Series>
            <telerik:PieSeries StartAngle="90">
                <%-- This is how it should be configured after the feature request --%>
                <%--<HighlightAppearance InactiveOpacity="0.2" />--%>

                <SeriesItems>
                    <telerik:PieSeriesItem Name="Slice1" Y="60"></telerik:PieSeriesItem>
                    <telerik:PieSeriesItem Name="Slice2" Y="40"></telerik:PieSeriesItem>
                </SeriesItems>
            </telerik:PieSeries>
        </Series>
    </PlotArea>
</telerik:RadHtmlChart>