We seem to be experiencing a lot of issues with the Plot Charts. The biggest problem is the slide is actually being modified so when we download it, it does not reflect what we uploaded. This pptx slide you will see the bottom axis has labels that are centered across 3 columns. However, the thumbnail gets modified as in the attached image, and the downloaded PPTX is also not correct.
I also attached the downloaded slide IT.Slide.201.pptx so you can see how it gets converted.