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?
But if we directly watch that, it would redraw the graph too often, right? (i.e. every time someone types a letter). I was talking about the axis fields and the format string...