The functionality is currently exposed only by the x-axis of the sparkliens. It would be a good idea to be exposed by the other series types as well.

For the time being you can configure the cross hair through the kendo widget:
<script>
			function pageLoad() {
				$find('RadHtmlChart1').get_kendoWidget().setOptions({categoryAxis:{crosshair:{ color: "green", width: 2, visible: true}}});
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
					</telerik:ColumnSeries>
					<telerik:LineSeries Name="Series 2">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="39" />
							<telerik:CategorySeriesItem Y="19" />
							<telerik:CategorySeriesItem Y="29" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<LabelsAppearance RotationAngle="33"></LabelsAppearance>
					<Items>
						<telerik:AxisItem LabelText="Item 1" />
						<telerik:AxisItem LabelText="Item 2" />
						<telerik:AxisItem LabelText="Item 3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>

increase line width when mouse hovers

For the time being the following events can be handled through the chartObject.
-drag event is fired when the plot area of the main chart is dragged.
-zoom event is fired when the mouse wheel is used in the plot area of the main chart.
-select event is fired when the range of the selector from the navigator pane is changed.

JavaScript:

    <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
        <script>
            function OnLoad(sender) {
                var chart = sender;
                chart._chartObject.bind("dragEnd", OnDragEnd);
                chart._chartObject.bind("selectEnd", OnSelectEnd);
                chart._chartObject.bind("zoom", OnZoom);
            }
            function OnDragEnd(e) {
                alert("OnDragEnd triggered: \nMin: " + e.axisRanges['mainAxis'].min + "\n Max: " + e.axisRanges['mainAxis'].max);
            }
            function OnSelectEnd(e) {
                alert("OnSelectEnd triggered: \nFrom: " + e.from + "\n To: " + e.to);
            }
            function OnZoom(e) {
            
                alert("OnZoom triggered: \nMin: " + e.axisRanges['mainAxis'].min + "\n Max: " + e.axisRanges['mainAxis'].max);
            }
        </script>
    </telerik:RadCodeBlock>
ASPX:

        <telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Layout="Stock" Width="800" Height="400"
            Transitions="true">
            <ClientEvents OnLoad="OnLoad" />
            <PlotArea>
                <XAxis Name="mainAxis" DataLabelsField="SellDate" Type="Date">
                </XAxis>
                <Series>
                    <telerik:ColumnSeries DataFieldY="SellQuantity">
                    </telerik:ColumnSeries>
                </Series>
            </PlotArea>
            <Navigator Visible="true">
                <XAxis Name="navAxis"></XAxis>
                <SelectionHint Visible="true" />
                <Series>
                    <telerik:AreaSeries DataFieldY="SellQuantity">
                    </telerik:AreaSeries>
                </Series>
            </Navigator>
        </telerik:RadHtmlChart>

C#:

    protected void Page_Load(object sender, EventArgs e)
    {
        RadHtmlChart1.DataSource = GetData();
        RadHtmlChart1.DataBind();
    }

    protected DataTable GetData()
    {
        DataTable dt = new DataTable();

        dt.Columns.Add("ID", typeof(int));
        dt.Columns.Add("SellQuantity", typeof(int));
        dt.Columns.Add("SellDate", typeof(DateTime));

        dt.Rows.Add(1, 2, new DateTime(2011, 06, 12));
        dt.Rows.Add(1, 5, new DateTime(2011, 12, 12));
        dt.Rows.Add(2, 6, new DateTime(2012, 06, 17));
        dt.Rows.Add(3, 4, new DateTime(2012, 09, 18));
        dt.Rows.Add(4, 7, new DateTime(2013, 03, 18));

        return dt;
    }

The property indicates whether the series name will be visible in the legend. For the time being the following workaround can be used:

JavaScript:

    <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
        <script>
            function pageLoad() {
                var chart = $find("<%=RadHtmlChart1.ClientID%>");
                chart.get_kendoWidget().options.series[0].visibleInLegend = false;
                chart.get_kendoWidget().options.series[1].visibleInLegend = true;
                chart.repaint();
            }
        </script>
    </telerik:RadCodeBlock>

ASPX:

        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries Name="Series 1">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="1" />
                            <telerik:CategorySeriesItem Y="2" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                    <telerik:ColumnSeries Name="Series 2">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="3" />
                            <telerik:CategorySeriesItem Y="4" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>

For the time being the following workaround can be used:    

Category Axis:

JavaScript:

  <script>
            function pageLoad() {
                var chart = $find("<%=RadHtmlChart1.ClientID%>");
                chart._chartObject.options.categoryAxis.labels.template = "Item: #=value#";
                chart.repaint();
            }
        </script>

ASPX:

       <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="300">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries Name="series 1">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="30" />
                            <telerik:CategorySeriesItem Y="10" />
                            <telerik:CategorySeriesItem Y="25" />
                            <telerik:CategorySeriesItem Y="20" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                </Series>
                <XAxis>
                    <Items>
                        <telerik:AxisItem LabelText="1" />
                        <telerik:AxisItem LabelText="2" />
                        <telerik:AxisItem LabelText="3" />
                        <telerik:AxisItem LabelText="4" />
                    </Items>
                </XAxis>
            </PlotArea>
        </telerik:RadHtmlChart>

Numeric Axis:
        <script>
            function changeValues(value) {
                var newValue;
                switch (value) {
                    case 1:
                        newValue = "AA";
                        break;
                    case 2:
                        newValue = "BB";
                        break;
                    case 3:
                        newValue = "CC";
                        break;
                    case 4:
                        newValue = "DD";
                        break;
                    default:
                        newValue = value;
                }
                return newValue;
            }
            function pageLoad() {
                var chart = $find("<%=RadHtmlChart1.ClientID%>");
                chart._chartObject.options.xAxis.labels.template = "#=changeValues(value)#";
                chart.repaint();
            }
        </script>
        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="300">
            <PlotArea>
                <Series>
                    <telerik:ScatterLineSeries Name="series 1">
                        <SeriesItems>
                            <telerik:ScatterSeriesItem Y="30" X="1" />
                            <telerik:ScatterSeriesItem Y="10" X="2" />
                            <telerik:ScatterSeriesItem Y="25" X="3" />
                            <telerik:ScatterSeriesItem Y="20" X="4" />
                        </SeriesItems>
                    </telerik:ScatterLineSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>

For the time being the following workaround can be used:

JavaScript:

    <script>
        function pageLoad() {
            var chart = $find("<%=RadHtmlChart1.ClientID%>");
            chart.get_kendoWidget().options.valueAxis.labels.template = "#if(value > 20) {# Value #=value# is a good option#} else {# value below 20 is a bad option #}#";
            chart.repaint();
        }
    </script>

ASPX:


        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="300">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries Name="series 1">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="30" />
                            <telerik:CategorySeriesItem Y="10" />
                            <telerik:CategorySeriesItem Y="25" />
                            <telerik:CategorySeriesItem Y="20" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                </Series>
                <YAxis>
                    <LabelsAppearance DataFormatString="dd{0}">
                    </LabelsAppearance>
                </YAxis>
                <XAxis>
                    <Items>
                        <telerik:AxisItem LabelText="item 1" />
                        <telerik:AxisItem LabelText="item 2" />
                        <telerik:AxisItem LabelText="item 3" />
                        <telerik:AxisItem LabelText="item 4" />
                    </Items>
                </XAxis>
            </PlotArea>
        </telerik:RadHtmlChart>

A nice improvement in RadHtmlChart will be the ability to control the speed of chart's transitions.

Allow the HTMLChart to be resized when the parent window is resized.
Currently the resize of the chart only occurs on page load or refresh only.

So i will resize the window and reload and the chart will fit into the space.

then if i enlarge the window the chart stays the current size.


This is particular important for us as our support of mobile devices (and responsive design)

the whole of our site is responsive apart from our charts :-( 

The options for positioning a legend in a RadHtmlChart are somewhat limited. Useful extensions:

1. Support Left/Center/Right options for legends positioned at top and bottom; support Top/Middle/Bottom options for legends positioned at left and right.

2. Support positioning legends within the plot area.

For the time being the following workaround can be used:
		<script>
			function pageLoad() {
				var kendoWidget1 = $find("<%=PieChart1.ClientID%>").get_kendoWidget();
				kendoWidget1.options.legend.position = "custom";
				kendoWidget1.options.legend.offsetX = 300;
				kendoWidget1.options.legend.offsetY = 80;
				kendoWidget1.redraw();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Transitions="true">
			<PlotArea>
				<Series>
					<telerik:PieSeries StartAngle="90">
						<LabelsAppearance Position="OutsideEnd" DataFormatString="{0} %" />
						<TooltipsAppearance DataFormatString="{0} %" />
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="Purple" Exploded="true" Name="Internet Explorer" Y="18.3" />
							<telerik:PieSeriesItem BackgroundColor="Orange" Exploded="false" Name="Firefox" Y="35.8" />
							<telerik:PieSeriesItem BackgroundColor="Green" Exploded="false" Name="Chrome" Y="38.3" />
							<telerik:PieSeriesItem BackgroundColor="Blue" Exploded="false" Name="Safari" Y="4.5" />
							<telerik:PieSeriesItem BackgroundColor="Red" Exploded="false" Name="Opera" Y="2.3" />
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
			<ChartTitle Text="Browser Usage for April 2012">
			</ChartTitle>
		</telerik:RadHtmlChart>

It would be great to be able to specify more layout properties via the data bound data. For example, if we could bind a property in the data to a series type we could generate different series on the fly.

This is important to me as I want to make the page as responsive as possible, so all charts load on demand - but as the data is generated in an asynchronous fashion, I'm not able to predetermine how many series I can create.

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 following workaround can be used:
JavaScript:
	<script>
		function pageLoad() {
			var pieChart = $find('<%=PieChart1.ClientID%>');
			pieChart._chartObject.options.seriesDefaults.labels.distance = 10;
			pieChart.repaint();
		}
	</script>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="PieChart1" Width="300" Height="300">
			<PlotArea>
				<Series>
					<telerik:PieSeries>
						<SeriesItems>
							<telerik:PieSeriesItem Name="Item 1" Y="18.3"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem Name="Item 2" Y="35.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem Name="Item 3" Y="38.3"></telerik:PieSeriesItem>
						</SeriesItems>
					</telerik:PieSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

When an item's value is equal to zero in a Bar or Column series,, its labels is not shown.

I would like the ability to set a gradient for the plot area of the RadHtmlChart control, instead of just being able to set a solid color.  Thanks!

Currently the RadHtmlChart can be bound to a single data source. Add the ability to bind each series to a single data source.

A good improvement in RadHtmlChart would be the introduction of Waterfall series.

Currently there is some space between the PlotArea and the starting/ending points of the LineSeries in RadHtmlChart.
To changethat behavior, so that it is similar to the one in AreaSeries  the justified property (https://docs.telerik.com/kendo-ui/api/javascript/dataviz/ui/chart/configuration/categoryaxis.justified) can be set through the underlying Kendo Chart widget:

        <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
            <script>
                function onLoad(sender) {
                    var chart = sender.get_kendoWidget();
                    var opts = chart.options;
                    opts.categoryAxis.justified = true;
                    chart.setOptions(opts);
                    chart.redraw();
                }
            </script>
        </telerik:RadCodeBlock>
        <telerik:RadHtmlChart runat="server" ID="LineChart" Width="800" Height="500" Transitions="true">
            <ClientEvents OnLoad="onLoad" />
            <PlotArea>
                <Series>
                    <telerik:LineSeries Name="Week 15" MissingValues="Interpolate">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="15" />
                            <telerik:CategorySeriesItem Y="23" />
                            <telerik:CategorySeriesItem />
                            <telerik:CategorySeriesItem Y="71" />
                            <telerik:CategorySeriesItem Y="93" />
                            <telerik:CategorySeriesItem Y="43" />
                            <telerik:CategorySeriesItem Y="23" />
                        </SeriesItems>
                    </telerik:LineSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>