Extend this plugin to show a graph of upgrade latency

[brendanheywood]

The idea here is we know when each release comes out, and we know when each release was upgraded to, so show a graph showing how out of date we are at any given day.

So a rough mockup, the top line is a 'good' site which might deploy within a couple days of release, and then its current for some time so most of the time its latency is 0. The bottom line is a less good site which doesn't release very often and so it constantly building up.

@paulholden if you are happy in principle with this we'll work on the patch?