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?
oooh ok I was thinking of the legendString, which is just an id and the format string, not the expression. I'll return a string that is the concatenation of the legend_id and the axis_id, that should be compared and trigger the redraw if either changes