Calculate total weeks based on first and last event
Only when both start and done are missing, treat as missing data

Otherwise, if one is present and zero, data is available and should plot
data in the graph.
Remove unused functions
Remove duplication when creating volumetric series
Merge branch 'master' into fix-missing-journey-data
Merge pull request #217 from alphagov/fix-missing-journey-data

Fix missing journey data