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?
Excellent. How about constructing a map `axisIdByExprId` before this call and only passing this in? The data transformer would only get the data it actually needs, and if expression lists ever get long, we have less id-searching to do (not that it matters for the small expression lists we have, but doing a `map()` is `O(n_expr^2)` :) ).