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
Also, given this is currently the *only* place where the `lastDefined` method is used, it's probably/possibly worth making its signature compatible with the usage rather than needing the `apply` dance.