Dim bm As New RenderTargetBitmap(CInt(c1Chart1.ActualWidth), CInt(c1Chart1.ActualHeight), 96, 96, PixelFormats.[Default])
bm.Render(c1Chart1)
Dim enc As New PngBitmapEncoder()
enc.Frames.Add(BitmapFrame.Create(bm))
Dim fs As New FileStream("chart.png", FileMode.Create)
enc.Save(fs)

RenderTargetBitmap bm = new RenderTargetBitmap(
      (int)c1Chart1.ActualWidth,(int)c1Chart1.ActualHeight,
        96, 96, PixelFormats.Default);
bm.Render(c1Chart1);
PngBitmapEncoder enc = new PngBitmapEncoder();
enc.Frames.Add(BitmapFrame.Create(bm));
FileStream fs = new FileStream("chart.png", FileMode.Create);
enc.Save(fs);