Currently, the diagrams are images created by the server, so, as far as I know, there is no way to transfer a parameter that controls the background color, or change it after loading it.
Soon, however, we will add new charting capabilities based on the new Apache Analytics 2.0. They will be displayed on the client side and very customizable and should be able to do what you describe.
In the meantime, you should be able to retrieve data that supports these diagrams through the WSAPI.
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