That's close, but not quite what I'm looking for. The calculation you suggested is giving me the percentage of each advertiser within P&G and the percentage of each advertiser within Other.
I need a "P&G percentage of the whole" and an "Other percentage of the whole".
In your example, each orange shaded piece would have the same percentage, and each blue shaded piece would have the same percentage. Ultimately I'm using the calculation only in the title (not the labels), so I only need the percentage of the blue shaded pieces.