Add "spend by lot" grouped timeseries graph to G-Cloud dashboard

Pivotal: https://www.pivotaltracker.com/s/projects/911874/stories/62377934

There is a bug in the positioning of the line labels: and we need to add currency symbols to the graph labels. I would prefer to fix these while pairing, since I think two pairs of eyes will be better than one, especially on the latter. 

I've added a basic test to check the graph is rendering: please tell me if more are needed.