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?
`data[i].axis_id` was being cached between requests for new data from Prometheus. This meant that if a user changed the axis for an expression, redrawing the graph as a result would use the old cached axis value. The expression would not be correctly drawn on the new axis until new data was requested.

The change I made here is to find the expression that belongs to the data, and then use that expression's current `axis_id`. The expression will always be relevant to its cached data.