I have observed the requirements shared by you and like to share that it seems to be an issue while setting the plot area height. An issue with ID SLIDESNET-33803 has been created in our issue tracking system to further investigate and resolve the issue. This thread has been linked with the issue so that you may be automatically notified once the issue will be resolved.
I like to share that the issue shared has been resolved in Aspose.Slides for .NET 6.9.0. Please use the following code snippet to observe the values being applied.