Formatting negative values

Hi everyone,
I have a problem with formatting negative values. 
    I have number :   1234567,892 and I need format it:   12,334,567.892  
    I have number : - 1234567,892 and I need format it: - 12,334,567.892 
But my result when I use DataFormatString "###,###,###.###":
     1234567,892    => 12,334,567.892  : Correct
   - 123,892            => - ,123,892            : Not correct   (I need -123,892)
     123,892            =>    ,123,892            : Not correct   (I need  123,892)
Have you any ideas ?
Thanks you very much !

P/s: My code
 <telerik:RadHtmlChart runat="server" ID="rcProfile" Height="380px" PlotArea-Appearance-Visible="false">                                                
                                                <PlotArea>
                                                    <Appearance>
                                                        <FillStyle BackgroundColor="White" />                                                        
                                                    </Appearance>
                                                    <Series>
                                                        <telerik:ColumnSeries DataFieldY="VAL_P" Name="Công suất" Stacked="false" ColorField="Blue"
                                                            Spacing="0" Gap="0">
                                                            <Appearance>
                                                                <FillStyle BackgroundColor="#F0AD4E" />                                                                
                                                            </Appearance>
                                                            <LabelsAppearance Visible="false">
                                                                <TextStyle Color="Red" />
                                                            </LabelsAppearance>
                                                            <LabelsAppearance DataFormatString="###,###,###,###.###"></LabelsAppearance>
                                                            <TooltipsAppearance BackgroundColor="White" DataFormatString="###,###,###,###.###" />
                                                        </telerik:ColumnSeries>
                                                    </Series>
                                                    <XAxis DataLabelsField="GIO_HTHI" >
                                                        <MajorGridLines Visible="false" Color="Blue" />
                                                        <MinorGridLines Visible="false" Color="Blue" />
                                                        <LabelsAppearance RotationAngle="75">
                                                        </LabelsAppearance>
                                                        <TitleAppearance Text="Thời gian">
                                                            <TextStyle FontFamily="Time new roman"  />
                                                        </TitleAppearance>
                                                    </XAxis>
                                                    <YAxis>
                                                        <MinorGridLines Visible="false" />
                                                        <MajorGridLines Color="WhiteSmoke" />
                                                        <TitleAppearance Text="kWh">
                                                            <TextStyle FontFamily="Time new roman" />
                                                        </TitleAppearance>
                                                    </YAxis>
                                                </PlotArea>
                                                <Legend>
                                                    <Appearance Visible="false">
                                                    </Appearance>
                                                </Legend>
                                                <ChartTitle Text="BIỂU ĐỒ PHỤ TẢI">
                                                    <Appearance>
                                                        <TextStyle Bold="true" FontFamily="Time new roman" Color="Blue" />
                                                    </Appearance>
                                                </ChartTitle>
                                            </telerik:RadHtmlChart>