Neil P. Quinn, 25/08/2015 01:58:
Sorry—it turns out that this is a browser bug! All of the graphs except legacy pageviews display no lines at all in Firefox (I've tested 42.0a2 and 41.0b3). I really should have checked that first.
I'll file it in Phab; hopefully, you can take a look at some point.
It's already filed, please add/edit: https://phabricator.wikimedia.org/T109693
Nemo