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?
That's a good point. Watching `scope.graphSettings.expressions` won't be much more taxing, as each `expression` only has 2 properties.