Unable to capture WPF livechart in bitmap

Question

I'm trying to write a function that will write the results of a test (which have both text output and chart output, stored in a textbox and a livecharts chart respectively) to a pdf file. There are no issues with the text output, but I haven't been able to get the charts to save unless I extract each chart from its parent stackpanel (each test has a stackpanel that is shown when the test is selected) and then display the chart in a separate window. There is a lot of code around the problematic lines, so I'm going to paste a shortened version below.

The issue seems to be calling "_saveWindow.Show()" instead of ".ShowDialog". I need this to open and close without user input, but the "Show()" doesn't draw the chart in the window at all.

Any ideas why this is happening?

foreach (TestSequenceItem tsi in resultsToSave)
{
     tsi.Instances[0].ChartStackPanel.Visibility = Visibility.Visible;

     for (int i=0; i<tsi.Instances[0].StackChartList.Count; i++)
     {
          Chart _saveChart = tsi.Instances[0].StackChartList[i];
          tsi.Instances[0].ChartStackPanel.Children.Remove(_saveChart);

          ScrollViewer panel = new ScrollViewer { Content = _saveChart };
          Window _saveWindow = new Window { Content = panel };

          _saveWindow.Show();

          var encoder = new PngBitmapEncoder();
          RenderTargetBitmap bmp = new RenderTargetBitmap((int)tsiChartDoc.DefaultPageSetup.PageWidth, 600, 96, 96, PixelFormats.Pbgra32);
          bmp.Render(_saveChart);
          encoder.Frames.Add(BitmapFrame.Create(bmp));

          using (MemoryStream stm = new MemoryStream())
          {
               encoder.Save(stm);

               string fileName = "base64:" + Convert.ToBase64String(stm.ToArray());

               // Adding a heading to the pdf
               tsiChartDoc.LastSection.AddParagraph(tsi.Instances[0].StackTitleList[i].Text, "Heading2");

               tsiChartDoc.LastSection.AddImage(fileName);
          }

          _saveWindow.Close();
          panel.Content = null;
                            
          tsi.Instances[0].ChartStackPanel.Children.Insert(chartIdx, _saveChart);
     }
}