Two small DAG chart ui fixes

[Ash Berlin-Taylor <as...@firemirror.com>]

I just opened two small PRs to fix some minor (but annoying) glitches on the DAG task duration page:

Task Duration shows two charts on first page load
https://issues.apache.org/jira/browse/AIRFLOW-1741 

NVD3 Charts do not have labeled axes and units change dynamically
https://issues.apache.org/jira/browse/AIRFLOW-1432

I have set a fix version of the one I created (1741) to 1.10 currently. As I'm not an admin I can't set fix version on the other. 1741 doesn't really affect much but without 1432 its hard to work out what the units are on the chart - is it seconds, minutes, or hours?

I think both of these were regressions from the switch from Highline to NVD3 charts (around 1.8?). I'd like them in 1.9, but we can always carry a patch for these internally until 1.10 is released :)

-ash