How to save chart generated in WinRT (Windows Store)

Question

I generated a chart using WinRT Toolkit. I want to save this chart to png file or other image file. I have using a WriteableBitmapSaveExtensions but it works like my current code:

MemoryStream ms = await WinRTXamlToolkit.Composition.WriteableBitmapRenderExtensions.RenderToPngStream(chart);
Windows.Storage.Pickers.FileSavePicker savePicker = new Windows.Storage.Pickers.FileSavePicker();
savePicker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.PicturesLibrary;
savePicker.FileTypeChoices.Add("JPG file", new List<string>() { ".png" });
Windows.Storage.StorageFile file = await savePicker.PickSaveFileAsync();
using (Stream x = await file.OpenStreamForWriteAsync())
      {
            x.Seek(0, SeekOrigin.Begin);
            ms.WriteTo(x);
      }

Problem with this code is saving chart without lines! For example this is chart from my win store app:

and my png file is:

Can someone help me with this strage thing?

Best Regards, Matt.