Add "toasts" explaining how log-ratios are set?

[fedarko]

Something like "Set log-ratio from clicking twice on the rank plot." could be useful, I think, esp. for new users.

We could also do this for intermediate clicks on the rank plot -- for example "set numerator of a log-ratio; click again to select the denominator" would pop up after the first click. (Would explain clicking better, based on discussion with @mestaki.)

This would have to be super non-intrusive and not annoying. Bootstrap has this sorta functionality available...