Saints preserve us. OK, as you haven't specified a chart component, and given that my previous answer to you lead to a conversation that was an exercise in complete futility, here's the code I would use to save a visual element out to file.
int chartWidth = (int)chart.ActualWidth;
int chartHeight = (int)chart.ActualHeight;
double resolution = 96d;
RenderTargetBitmap bmp = new RenderTargetBitmap(chartWidth, chartHeight, resolution, resolution, PixelFormats.Pbgra32);
bmp.Render(chart);
BmpBitmapEncoder encoder = new BmpBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(bmp));
encoder.Save(...my file stream ...);
I've left out the code for creating the stream - you really should be able to figure that one out for yourself, and I've made assumptions that you want a 96x96 DPI image.