String data source fields in the RadHtmlChart that contain only numeric characters should not be parsed to decimal. Also when such a string starts with the 0 character, it is omitted. For example the "0" item in the legend is not displayed with the example below:
ASPX:

        <telerik:RadHtmlChart runat="server" ID="PieChart1" Width="800" Height="500" Transitions="true">
            <Appearance>
                <FillStyle BackgroundColor="White"></FillStyle>
            </Appearance>
            <ChartTitle Text="Browser Usage for April 2012">
                <Appearance Align="Center" BackgroundColor="White" Position="Top">
                </Appearance>
            </ChartTitle>
            <Legend>
                <Appearance BackgroundColor="White" Position="Right" Visible="true">
                </Appearance>
            </Legend>
            <PlotArea>
                <Appearance>
                    <FillStyle BackgroundColor="White"></FillStyle>
                </Appearance>
                <Series>
                    <telerik:PieSeries StartAngle="90" DataFieldY="code_count" NameField="items_names">
                        <LabelsAppearance Position="OutsideEnd" DataFormatString="{0} %">
                        </LabelsAppearance>
                        <TooltipsAppearance Color="White" DataFormatString="{0} %"></TooltipsAppearance>
                    </telerik:PieSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>

C#:

using System.Data;

    protected void Page_Load(object sender, EventArgs e)
    {
        PieChart1.DataSource = GetData();
        PieChart1.DataBind();
    }

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

        dt.Columns.Add("ID", typeof(int));
        dt.Columns.Add("code_count", typeof(int));
        dt.Columns.Add("items_names", typeof(string));

        dt.Rows.Add(1, 771, "0");
        dt.Rows.Add(2, 187, "1");
        dt.Rows.Add(3, 178, "2");
        dt.Rows.Add(4, 60, "3");

        return dt;
    }

Currently the datasource columns which have space in their name cannot be passed to a databound RadHtmlChart. Add the ability to handle the spaces in the names of these columns.

Currently it is not possible to add null-able series items in RadHtmlChart in the markup or programmatically. If a null-able item is added it is omitted and is replaced by the next series item from the current Series. The workaround is to either data bound the chart to a data source or convert the null-able values to zeros:

C#:

        decimal? nullValue = null;
        if (nullValue == null)
        {
            nullValue = 0;
        }

        ColumnSeries colSeries2 = RadHtmlChart1.PlotArea.Series[1] as ColumnSeries;
        colSeries2.SeriesItems.Add(39);
        colSeries2.SeriesItems.Add(nullValue);
        colSeries2.SeriesItems.Add(3);

The selection (http://demos.telerik.com/kendo-ui/chart-api/selection) can be configured through the kendo widget:
		<script>
			function pageLoad(args) {
				var kendoWidget = $find("<%=RadHtmlChart1.ClientID%>").get_kendoWidget();
				kendoWidget.options.categoryAxis.select = { from: 1, to: 2 };
				kendoWidget.bind("selectStart", onSelectStart);
				kendoWidget.bind("select", onSelect);
				kendoWidget.bind("selectEnd", onSelectEnd);

				kendoWidget.redraw();
			}

			function formatRange(e) {
				var categories = e.axis.categories;

				return kendo.format("{0} - {1} ({2} - {3})",
					e.from, e.to,
					categories[e.from],
					// The last category included in the selection is at (to - 1)
					categories[e.to - 1]
				);
			}

			function onSelectStart(e) {
				console.log(kendo.format("Select start :: {0}", formatRange(e)));
			}

			function onSelect(e) {
				console.log(kendo.format("Select :: {0}", formatRange(e)));
			}

			function onSelectEnd(e) {
				console.log(kendo.format("Select end :: {0}", formatRange(e)));
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="1" />
							<telerik:CategorySeriesItem Y="2" />
							<telerik:CategorySeriesItem Y="3" />
							<telerik:CategorySeriesItem Y="4" />
							<telerik:CategorySeriesItem Y="2" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
						<telerik:AxisItem LabelText="4" />
						<telerik:AxisItem LabelText="5" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>

For the time being you can workaround this behavior as follows:
  1)Place all of the items within a common series.
  2)Use different colors for items in order to distinguish them from their original series.
  3)Create a custom legend if needed one.

For the time being the following workaround can be used:

    <style>
        .k-chart .k-tooltip, .k-sparkline .k-tooltip, .k-stockchart .k-tooltip {
            background-image: url("//:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAWCAYAAADAQbwGAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADNJREFUeNpi/P//vwMDFQELEP8beQb+HTWQYgP/DHoD/466cAR4edRAyg38P6hLbIAAAwCnWhhVsxvdCAAAAABJRU5ErkJggg==") !important;
        }
    </style>

There is no axis click event. If we expose such event we can give custom info to the client about this axis. For the time being the event can the attached through the chartObject:
		<script>
			function pageLoad() {
				var chart = $find("<%=ColumnChart1.ClientID%>");
				chart._chartObject.bind("axisLabelClick", chart_axisLabelClick);
			}
			function chart_axisLabelClick(e) {
				alert(e.value);
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="15000" />
							<telerik:CategorySeriesItem Y="23000" />
							<telerik:CategorySeriesItem Y="10000" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="Item 1" />
						<telerik:AxisItem LabelText="Item 2" />
						<telerik:AxisItem LabelText="Item 3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>

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 a long legend in RadHtmlChart overflows the PlotArea under IE, while in the rest browsers it is cut. The workaround is to hide the legend and create a custom one in a styled div (e.g. KendoUI demonstrates such an example here - http://demos.kendoui.com/dataviz/bubble-charts/index.html)

Issue will be fixed for Q3 2014 release.
For the time being you can manually set color to the selection hint:
CSS:
	<style>
		.k-chart .k-navigator-hint .k-tooltip {
			color: black;
		}
	</style>

Reproduction steps - Run the code below and click Show Chart - chart is not displayed.
ASPX:
		<div id="div1" style="visibility: hidden;">
			<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Width="600px" Height="400px">
				<PlotArea>
					<Series>
						<telerik:ColumnSeries Name="Product 1">
							<SeriesItems>
								<telerik:CategorySeriesItem Y="1" />
								<telerik:CategorySeriesItem Y="2" />
								<telerik:CategorySeriesItem Y="3" />
							</SeriesItems>
						</telerik:ColumnSeries>
					</Series>
				</PlotArea>
			</telerik:RadHtmlChart>
		</div>
		<div id="chart" style="visibility: hidden;">HtmlChart wrapper</div>
		<input type="button" onclick="showChart(); return false;" value="Show Chart" />
		<script>
			$ = $telerik.$;
			function showChart() {
				$("#div1").attr("style", "visibility:visible");
			}
		</script>
Solution 1:
Use the workaround below:
		<telerik:RadRotator ID="RadRotator1" runat="server" Height="500px" ItemHeight="300px" ItemWidth="300px" Skin="Default" Width="1000px" RotatorType="AutomaticAdvance">
			<Items>
				<telerik:RadRotatorItem>
					<ItemTemplate>
						<telerik:RadHtmlChart ID="RadHtmlChart2" runat="server" Height="300px" Skin="Silk" Width="300px" Transitions="False" Visible="true">
							<PlotArea>
								<Series>
									<telerik:PieSeries>
										<SeriesItems>
											<telerik:PieSeriesItem Name="a" Y="12" />
											<telerik:PieSeriesItem Name="b" Y="5" />
										</SeriesItems>
									</telerik:PieSeries>
								</Series>
							</PlotArea>
						</telerik:RadHtmlChart>
					</ItemTemplate>
				</telerik:RadRotatorItem>
			</Items>
			<ItemTemplate>
			</ItemTemplate>
		</telerik:RadRotator>
		<script>
			$ = $telerik.$;
			Telerik.Web.UI.RadHtmlChart.prototype.repaint = function () {
				var that = this;
				if (that._chartObject) {
					if (that._isDataSourceChanged()) {
						that._loadData();
					}
					else {
						that._resetVisibilityOption();
						that._chartObject.redraw();
					}
				}
			};

			Telerik.Web.UI.RadHtmlChart.prototype._resetVisibilityOption = function () {
				var that = this;
				that._chartObject.options.visible = that._options.visible = that.get_visible();
			};

			Telerik.Web.UI.RadHtmlChart.prototype._getMainConfig = function () {
				var that = this;
				//when visible is false - nothing is sent from the server to save transferred data
				//e.g - there are properties set but visible is false - we do not need to send those properties amyway
				//that is why - set visible: false when nothing is sent from the server
				that._options = $telerik._getPropertiesParameter(that, that.constructor);
				that._cleanUpConfigProperties();

				that._options["theme"] = that.get_skin();
				that._options["visible"] = $(that.get_element()).is(':visible');
				that._options["title"] = that._chartTitle ? that._parseObject(that._chartTitle) : { visible: false };
				that._options["legend"] = that._legend ? that._parseObject(that._legend) : { visible: false };

				if (that._chartArea) {
					that._options["chartArea"] = that._parseObject(that._chartArea);
				}

				if (that._isSparklineChart()) {
					that._setMinimumSparklineWidth();
				}

				if (that._plotArea) {
					that._configurePlotArea();
				}

				if (that._series) {
					/*jshint evil: true */
					that._options["series"] = eval(that._series);
					/*jshint evil: false */
				}

				if (that._isStockChart()) {
					that._configureStockChart();
				}
			};
		</script>
Solution 2:
Hide the chart on load event:
		<div id="div1">
			<telerik:RadHtmlChart runat="server" ID="ColumnChart1" Width="600px" Height="400px">
				<ClientEvents OnLoad="OnLoad" />
				<PlotArea>
					<Series>
						<telerik:ColumnSeries Name="Product 1">
							<SeriesItems>
								<telerik:CategorySeriesItem Y="1" />
								<telerik:CategorySeriesItem Y="2" />
								<telerik:CategorySeriesItem Y="3" />
							</SeriesItems>
						</telerik:ColumnSeries>
					</Series>
				</PlotArea>
			</telerik:RadHtmlChart>
		</div>
		<div id="chart" style="visibility: hidden;">HtmlChart wrapper</div>
		<input type="button" onclick="showChart(); return false;" value="Show Chart" />
		<script>
			$ = $telerik.$;
			function showChart() {
				$("#div1").attr("style", "visibility:visible");
			}

			function OnLoad() {
				$("#div1").attr("style", "visibility:hidden");
			}

		</script>

For the time being the property can be set through the kendo widget:
		<script type="text/javascript">
			function pageLoad() {
				var kendoWidget = $find("<%=RadHtmlChart1.ClientID%>").get_kendoWidget();
				kendoWidget.options.series[0].color = function (e) {
					if (e.value > 20) {
						return "red";
					}
					else {
						return "green";
					}
				}
				kendoWidget.redraw();
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries>
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
					</telerik:ColumnSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>

Issue is reproducible with the following code:
        <telerik:RadHtmlChart runat="server" ID="ColumnChart1" Width="600px" Height="400px">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries Name="Product 1">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="10" />
                            <telerik:CategorySeriesItem Y="8" />
                        </SeriesItems>
                        <LabelsAppearance Position="Center"></LabelsAppearance>
                    </telerik:ColumnSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>
For the time being you can change the labels position from "Center" to "OutsideEnd".