Fix handling of empty data sets in completion collection

Calculations involve potential division by 0 if data sets are empty, which results in rendering out "NaN" to users.
Display the last valid model, rather than the total, of the collection for line labels

This was particularly bad for lines showing totals for other lines in the graph.

https://www.pivotaltracker.com/n/projects/911874/stories/74682680
Merge pull request #675 from alphagov/bugfix/completion-rate-no-data

Fix handling of empty data sets in completion collection
Merge pull request #676 from alphagov/feature/line-labels

Display the last valid model, rather than the total, of the collection for line labels