Hi James,
Thanks for contacting Syncfusion Support.
We have exported chart as image by using RenderTargetBitmap. If control background is “Transparent”, then output of control image background will be “Black” in RenderTargetBitmap. So, while applying the background for SfChart, it will get applied to SfChart by excluding its margin (margin parts of control also exported, but it is not visible due to black background).
However, we have achieved your requirement by placing the chart inside an additional container (Grid). We have prepared a demo sample for your requirement and it can be downloaded from below link,
Please find the screenshot of exported image
Regards,
Devi