Fix line labels for line graphs showing percentages.

Where line graphs show percentages, we want the lines to be
labelled.

However, we want the labels to show the last value on the line
- not the overall summed values and percentages, as we use
on stacked graphs.

Amend the way we calculate labels accordingly.

Story: https://www.pivotaltracker.com/s/projects/911874/stories/72594902