Gene view: when comparing two time points, meaning of "tag" needs to be clearer

[ppavlidis]

It needs a '?' or a tooltip to say that the red tag is the first time point, purple is the second, etc.

Also, that ability to compare two time points is not documented. Add "Command-click to select a second time point for comparison" to the text at the top of the graph." (obv. say what to do for PC, Mac, Linux)