Support transition to proportions for multiple selections [feature request]

[UNFPAmaldives]

It would be neat if it were possible to select more than one column (or row) at a time, in order to view proportions relative to one another at T1, T2, etc.

[EE2dev]

Thanks for the feature request!

I have to think some more about the best concept how to do it - the hardest part seems to be avoiding overlap of the text labels. Not sure if there is an easy way to completely solve this.