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;
    }