Removed drawCursorLine from the app.

This was part of line and is no longer required.
Allow user to hover over parts of the graph with no-data.

This will now show the line (and no data). Before it would select the nearest model.

There are bugs around the line labels that need to be addressed seperatly to this.
Merge pull request #644 from alphagov/bugfix/no-data-line

Bugfix/no data line