WPF控件保存为图片

<MetroChart:ClusteredBarChart
                Name="PerformanceChart"
                ChartTitle="员工绩效"
                ChartSubTitle="流出流入绩效"
                FontSize="14"
                SelectedItem="{Binding Path=SelectedItem, Mode=TwoWay}"
                SeriesSource="{Binding Path=Series}"
                IsRowColumnSwitched="True"
             >
                <MetroChart:ClusteredBarChart.Resources>
                    <ResourceDictionary Source="pack://application:,,,/De.TorstenMandelkow.MetroChart;component/Themes/Generic.xaml"/>
                </MetroChart:ClusteredBarChart.Resources>
                <MetroChart:ClusteredBarChart.SeriesTemplate>
                    <DataTemplate>
                        <MetroChart:ChartSeries
                            SeriesTitle="{Binding Path=DisplayName}"
                            DisplayMember="Performer"
                            ValueMember="Score"
                            ItemsSource="{Binding Path=Items}"
                          />
                    </DataTemplate>
                </MetroChart:ClusteredBarChart.SeriesTemplate>
            </MetroChart:ClusteredBarChart>

 Size size = new Size(this.Width, this.Height);
            PerformanceChart.Measure(size);

            var rtb = new RenderTargetBitmap(
               (int)PerformanceChart.ActualWidth, //width
               (int)PerformanceChart.ActualHeight, //height
               96, //dpi x
               96, //dpi y
               PixelFormats.Pbgra32 // pixelformat
               );
            rtb.Render(PerformanceChart);         

            var bitmap = rtb;

            byte[] data;
            System.Windows.Media.Imaging.JpegBitmapEncoder encoder = new System.Windows.Media.Imaging.JpegBitmapEncoder();
            encoder.Frames.Add(System.Windows.Media.Imaging.BitmapFrame.Create(bitmap));
            using (System.IO.MemoryStream ms = new System.IO.MemoryStream())
            {
                encoder.Save(ms);
                data = ms.ToArray();
            }

            NPOI.HSSF.UserModel.HSSFWorkbook workbook = new NPOI.HSSF.UserModel.HSSFWorkbook();
            var sheet = workbook.CreateSheet();
            var row = sheet.CreateRow(0);
            var cell = row.CreateCell(0);
            cell.SetCellValue("Hello world");
            row = sheet.CreateRow(1);
            cell = row.CreateCell(0);

            int pictureIdx = workbook.AddPicture(data, NPOI.SS.UserModel.PictureType.PNG);

            NPOI.HSSF.UserModel.HSSFPatriarch patriarch = (NPOI.HSSF.UserModel.HSSFPatriarch)sheet.CreateDrawingPatriarch();
            var rowsNum = 1;
            //add a picture
            NPOI.HSSF.UserModel.HSSFClientAnchor anchor = new NPOI.HSSF.UserModel.HSSFClientAnchor(0, 10, 1023, 0, 0, rowsNum, 2, rowsNum);

            NPOI.HSSF.UserModel.HSSFPicture pict = (NPOI.HSSF.UserModel.HSSFPicture)patriarch.CreatePicture(anchor, pictureIdx);
            pict.Resize();

            var dlg = new Microsoft.Win32.SaveFileDialog { Filter = ".xls files|*.xls", DefaultExt = ".xls" };
            var path = dlg.ShowDialog(this).Value ? dlg.FileName : null;
            if (path != null)
            {
                using (System.IO.FileStream fs = new System.IO.FileStream(dlg.FileName, System.IO.FileMode.OpenOrCreate))
                {

                    //将内容写入到硬盘中 

                    workbook.Write(fs);

                }
            }
            System.Windows.MessageBox.Show("导出成功！");