Thanks for considering Aspose.Slides.
I regret to inform you that issue ID 14471 is still in progress. As far as chart rendering issue in the generated PPTX slide thumbnail is concerned, the rendering of charts is currently not supported in Aspose.Slides for .NET. The issue is in development phase and will be available in the next 3-4 months. At first, we will provide API for charts access and only after that their rendering.
For chart rendering issue in PPTX a separate issue with ID 16980 has already been created in our issue tracking system. I am associating this thread with the mentioned issues so that you may be automatically notified, once these issues are resolved.
We are sorry for your inconvenience and really appreciate your patience,