Comparing time tracked on multiple (sets of) tags

[almarklein]

From a user:

Maybe a way to compare two separate sets of tags, like comparing how much #a #b vs #c #d has been logged during a given period. This could pop up under the main box in the overview section as a second entry to select which tags to compare with... although I see how the left 'records view' would need some way to be resolved (maybe with each box being focused?)

It's an interesting idea, but it would need some thought how to make this work - it does not fit well into anything that timetagger does already, so it would be a new visualization option somehow.

Related to #125. Maybe we need some sort of mode to compose visualizations in an interactive manner.