Stop redrawing graph on each expression change.
Add $watch for legend_id and legendStr.name to dynamically upgrade chart
when a legend string is modified, or a different format string is
applied.
Fix bug where changing expression axis would not update the chart.

axis_id was only being assigned when querying new data from Prometheus.
This has been changed so that each time the graph is redrawn, each data
series is assigned its current axis_id.
Merge pull request #188 from prometheus/fix-expression-dropdown

Stop redrawing graph on each expression change; fix bug where changing graph axis does not update the graph.