Stop redrawing graph on each expression change.

Check the typeahead performance on this @juliusv. I realized this weekend that `redrawGraph` was being fired each time an expression changed, which would explain the poor performance. I also don't see any reason for that to be there, right?
Hmm, shouldn't this watch the axis selector too? Otherwise selecting an axis requires a manual refresh.

I'm not sure about replacing the `graphSettings.legendFormatStrings` watch with a function watch. Does it improve anything? Shouldn't almost any edit to `graphSettings.legendFormatStrings` also change the resulting legend name?