A bug of the Android PDF-API forced us to switch to a different PDF-API. However creating a screenshot and print it to the PDF should do the job. For all i know we use this feature in the iOS-Version of our app.
The difficulty is to render the charts in the background, out of the user's view. Unfortunately i do not know the exact procedure.
Finally I could solve the problem, by developing a small, simple diagram-framework on my own, that uses the primitive drawing-functions of the respective PDF-framework. Since there is no need of any user-interaction, like zooming and swiping, and no dynamic behaviour is required, this can be done easily.
Hope this answer helps You to find a solution