Charts are not preserved when saving to PDF. I use Gembox for conversion to PDF only and for me this would be a very nice feature. It would not be a problem if only a subset of chart-types are supported. The most basic ones are most used anyway.
Also, it should use the cached values, or at least support using these values, since those are the ones that my application currently modifies when rendering charts to documents.
This feature request has been implemented and is available in the latest versions of GemBox.Document.