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?
`""` causes the `expr.legend_id` to be cast to a string, and then added to `expr.axis_id`. This is necessary because if `legend_id` isn't set, `expr.legend_id + expr.axis_id` = `NaN`.