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;
    }

Currently MinValue and MaxValue properties of the Axes accepts only decimal, so that when the axes are of date type, Min and Max dates cannot be set.

The workaround is to obtain the min and max values from the datasource field used for populating XAxis labels and set these values explicitly to the XAxis through the chartObject. For example:
<script type="text/javascript">
    function pageLoad() {
        //Get reference to the chart
        var chart = $find("<%=RadHtmlChart1.ClientID%>");
  
        //Obtain charts datasource
        var currDataSource = eval(chart.get_dataSourceJSON());
  
         //Get minValue for the datasource field used for populating XAxis Labels. In this example the field is named myDataSourceField
        var minValue = currDataSource[0].myDataSourceField;
  
        //Get maxValue for the datasource field used for populating XAxis Labels. In this example the field is named myDataSourceField
        var maxValue = currDataSource[currDataSource.length - 1].myDataSourceField;
  
        //Reference the chartObject's min and max properties
        chart._chartObject.options.xAxis.min = minValue;
        chart._chartObject.options.xAxis.max = maxValue;
  
        //Repaint the chart
        chart.repaint();
    }
</script>

For the time being you can utilize the Kendo UI charting scripts (http://demos.telerik.com/kendo-ui/bullet-charts/index) and API that RadHtmlChart use, in order to render a bullet chart:
	<form id="form1" runat="server">
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			function createChart() {
				$telerik.$("#chart").kendoChart({
					legend: {
						visible: false
					},
					series: [{
						type: "bullet",
						data: [[750, 762.5]]
					}],
					chartArea: {
						margin: {
							left: 0
						}
					},
					categoryAxis: {
						majorGridLines: {
							visible: false
						},
						majorTicks: {
							visible: false
						}
					},
					valueAxis: [{
						plotBands: [{
							from: 715, to: 752, color: "#ccc", opacity: .6
						}, {
							from: 752, to: 772, color: "#ccc", opacity: .3
						}],
						majorGridLines: {
							visible: false
						},
						min: 715,
						max: 795,
						minorTicks: {
							visible: true
						}
					}],
					tooltip: {
						visible: true,
						template: "Maximum: #= value.target # <br /> Average: #= value.current #"
					}
				});
			}
			function pageLoad() {
				createChart();
			}
		</script>
		<div id="chart"></div>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" style="display:none;"></telerik:RadHtmlChart>
	</form>

A JS error is thrown when a RadHtmlChart is databound and there are series items with null values that correspond to DateTime values on the XAxis. 

The issue could be handled in the following way:
Check if there is a series item's value that is null and if this is the case then do not add it to the datasource

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>

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!

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>

For the time being you can choose either approach:

1) Set the min and max dates on the kendo widget:
ASPX:
		<script>
			function pageLoad() {
				var chart = $find("<%=RadHtmlChart1.ClientID%>");
				chart._chartObject.options.xAxis.min = new Date(2013, 04, 01);
				chart._chartObject.options.xAxis.max = new Date(2013, 05, 01);
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="640px" Height="480px">
			<PlotArea>
				<Series>
					<telerik:ScatterLineSeries DataFieldY="SellQuantity" DataFieldX="SellDate">
						<LabelsAppearance DataFormatString="{1} cars sold on {0:m}">
						</LabelsAppearance>
						<TooltipsAppearance Color="White" DataFormatString="{1} cars sold on<br/>{0:D}" />
					</telerik:ScatterLineSeries>
				</Series>
				<XAxis BaseUnit="Days">
					<TitleAppearance Text="Sell Date">
					</TitleAppearance>
					<LabelsAppearance DataFormatString="d" RotationAngle="45">
					</LabelsAppearance>
					<MajorGridLines Color="#EFEFEF" Width="1"></MajorGridLines>
					<MinorGridLines Color="#F7F7F7" Width="1"></MinorGridLines>
				</XAxis>
				<YAxis>
					<TitleAppearance Text="Quantity">
					</TitleAppearance>
					<MajorGridLines Color="#EFEFEF" Width="1"></MajorGridLines>
					<MinorGridLines Color="#F7F7F7" Width="1"></MinorGridLines>
				</YAxis>
			</PlotArea>
			<ChartTitle Text="Sold Cars per Date">
			</ChartTitle>
		</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(2013, 05, 12));
		dt.Rows.Add(2, 5, new DateTime(2013, 05, 13));
		dt.Rows.Add(3, 6, new DateTime(2013, 05, 17));
		dt.Rows.Add(4, 4, new DateTime(2013, 05, 18));
		dt.Rows.Add(5, 7, new DateTime(2013, 05, 22));

		return dt;
	}

2) Set the JavaScript equivalent of min and max dates in the code behind like follows:
C#:
	protected void Page_Load(object sender, EventArgs e)
	{
		//Set min and max date for a datetime x-axis type
		RadHtmlChart1.PlotArea.XAxis.MinValue = ConvertToJavaScriptDateTime(new DateTime(2014, 02, 19));
		RadHtmlChart1.PlotArea.XAxis.MaxValue = ConvertToJavaScriptDateTime(new DateTime(2014, 04, 05));

		//Instantiate ScatterSeriesItem objects
		ScatterSeriesItem ssi1 = new ScatterSeriesItem();
		ScatterSeriesItem ssi2 = new ScatterSeriesItem();
		ScatterSeriesItem ssi3 = new ScatterSeriesItem();
		ScatterSeriesItem ssi4 = new ScatterSeriesItem();
		//Instantiate DateTime objects
		DateTime date1 = (new DateTime(2014, 03, 03, 0, 0, 0));
		DateTime date2 = (new DateTime(2014, 03, 10, 0, 0, 0));
		DateTime date3 = (new DateTime(2014, 03, 17, 0, 0, 0));
		DateTime date4 = (new DateTime(2014, 03, 24, 0, 0, 0));
		//Set the converted x values to the ScatterSeriesItem objects
		ssi1.X = ConvertToJavaScriptDateTime(date1);
		ssi2.X = ConvertToJavaScriptDateTime(date2);
		ssi3.X = ConvertToJavaScriptDateTime(date3);
		ssi4.X = ConvertToJavaScriptDateTime(date4);
		//Set the y values to the ScatterSeriesItem objects
		ssi1.Y = 3;
		ssi2.Y = 6;
		ssi3.Y = 5;
		ssi4.Y = 9;
		//Add the ScatterSeriesItem objects to the ScatterLineSeries
		(RadHtmlChart1.PlotArea.Series[0] as ScatterLineSeries).SeriesItems.Add(ssi1);
		(RadHtmlChart1.PlotArea.Series[0] as ScatterLineSeries).SeriesItems.Add(ssi2);
		(RadHtmlChart1.PlotArea.Series[0] as ScatterLineSeries).SeriesItems.Add(ssi3);
		(RadHtmlChart1.PlotArea.Series[0] as ScatterLineSeries).SeriesItems.Add(ssi4);
	}

	//A method that converts the .NET DateTime object to its JavaScript Date object representation
	protected decimal ConvertToJavaScriptDateTime(DateTime fromDate)
	{
		return (decimal)fromDate.Subtract(new DateTime(1970, 1, 1)).TotalMilliseconds;
	}
ASPX:
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="640px" Height="480px">
			<PlotArea>
				<Series>
					<telerik:ScatterLineSeries Name="Stock A">
						<LabelsAppearance>
							<ClientTemplate>
                        #= kendo.format(\'{0:d/MM/yyyy}\', new Date(value.x)) #,  #=kendo.format(\'{0:C0}\',value.y)#
							</ClientTemplate>
						</LabelsAppearance>
						<TooltipsAppearance Color="White">
							<ClientTemplate>
                        #= kendo.format(\'{0:d/MM/yyyy}\', new Date(value.x)) #,  #=kendo.format(\'{0:C0}\',value.y)#
							</ClientTemplate>
						</TooltipsAppearance>
					</telerik:ScatterLineSeries>
				</Series>
				<YAxis>
					<LabelsAppearance DataFormatString="C0"></LabelsAppearance>
					<TitleAppearance Text="Price"></TitleAppearance>
				</YAxis>
				<XAxis Type="Date" BaseUnit="Days">
					<TitleAppearance Text="Closing Date">
					</TitleAppearance>
					<LabelsAppearance DataFormatString="d">
					</LabelsAppearance>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Closing Stock Prices">
			</ChartTitle>
		</telerik:RadHtmlChart>

Width and height properties for the legend that will provide firm dimensions in pixels, so regardless of the series names' length, it will always have constant size.
You can set width and height through the kendo widget:
		<script>
			function pageLoad() {
				var chart = $find("<%=DonutChart.ClientID%>").get_kendoWidget();
				chart.options.legend.height = 80;
				chart.redraw();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="DonutChart" Width="500" Height="500" Transitions="true">
			<Appearance>
				<FillStyle BackgroundColor="White"></FillStyle>
			</Appearance>
			<ChartTitle Text="OS Usage Statistics for December 2012">
				<Appearance Align="Center" Position="Top"></Appearance>
			</ChartTitle>
			<PlotArea>
				<Series>
					<telerik:DonutSeries>
						<LabelsAppearance Visible="false">
						</LabelsAppearance>
						<TooltipsAppearance DataFormatString="{0}%" BackgroundColor="White"></TooltipsAppearance>
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="#00adcc" Name="Win7"
								Y="55.6"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#cccccc" Name="Win8" Y="2.5"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#999999" Name="Vista" Y="2.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#888888" Name="NT" Y="1.8"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#777777" Name="WinXP" Y="21.1"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#666666" Name="Linux" Y="4.7"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#555555" Name="Mac" Y="8.7"></telerik:PieSeriesItem>
							<telerik:PieSeriesItem BackgroundColor="#444444" Name="Mobile" Y="2.2"></telerik:PieSeriesItem>
						</SeriesItems>
					</telerik:DonutSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

Axes themselves do not have tooltips, only series do. Thus, tooltips cannot be added to the y-axis.
Add

I would like to request a new feature in legend of RadHtmlChart that provide a tool-tip for long names because when I add a long text in legends the size of pie-chart is reduced. 
Sample code for what I expect to be working :

<telerik:RadHtmlChart runat="server" ID="Chart" Height="400px" Width="400px">
<ClientEvents OnLoad="onChartLoad" />
function onChartLoad()
{
var chart = $find("Chart");
var widget = chart.get_kendoWidget();
widget.options.legend.tooltip = "#: dataItem.columnname #" //Feature Request in this line
widget.redraw();
}

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>

The event should be similar to OnClientSeriesClicked event. It should expose information about the clicked series.

Adding a Radar chart type will be a nice improvement in RadHtmlChart.

My objective is to compare product revenue over time. This I would like to achieve using data binding. I do not have information of product names or have the product names as columns. My data table structure is having three columns:

Date
Product Name
Revenue

This I should simply be able to achieve using data binding to a table with above three columns. 
In result I expect each of the Product Names to appear as independent series with revenues plotted on different dates.

A similar graph is displayed in the Line Chart documentation for RadHtmlChart.
https://docs.telerik.com/devtools/aspnet-ajax/controls/htmlchart/chart-types/line-chart

Currently BarSeries and ColumnSeries can be stacked in only one stack. Add the ability to group the stacked bars/columns Series through additional property (e. g StackName) for each Series in different stacks.

Currently when numerous series items are present, numerical series (Scatter/ScatterLine) calculates automatically the step through which the major/minor gridlines and the corresponding xaxis labels are rendered, so that they are visually readable.

For categorical charts, however, when multiple series items are present, all the xaxis labels are plotted, so that they are not readable. Add ability to autoadjust the step for the xaxis labels in categorical charts.