Write failing tests for bug with graphs being sleected when off to the left of data
Refactor `Line#getDistanceAndClosestModel`

The previous version of this method was an insane mess of spaghetti code.
In particular the option to handle null values was redundant because it was set to true everywhere.
Remove places where the `allowMissingData` option was passed

It's not used for anything any more
Use correct method to iterate on Backbone collection
Check for undefined-ness instead of NaN-ness

`getDistanceAndClosestModel` no longer returns NaN for missing values
*Technically* `isNaN(undefined) === true` but that would be a horror hack.
Use underscore meethods instead of nonsense
Trailing whitespace auto-removal
Removed test for options that is no longer used
Fix bug where graph points with zero y-positions do not return line diffs

Testing for truthiness instead of numberiness causes the diffs to not be calculated when the y-position for the closest point is zero
Merge pull request #379 from alphagov/bugfix/grouped-timeseries-hover-states

Bugfix/grouped timeseries hover states