fix issue of pie chart being invisible when total group value is 0

see issue #430

This solves 99% of the issue. I don't know how the title and label should be handled.

Ideally, we would like the user to be able to provide custom messages but how?
Currently, it uses the standard title and label functions, however the user might not be aware of this special case.
But providing additional title and label functions only for this case would add too much noise.