Plotting large datasets is too slow and crashes the app

[staticf0x]

No problem using ./manage.py generate_charts, but when it comes to Heroku, the limit is - I believe - 30s for a request and that's not enough for uploads with more than a year of data. Most of the time is spent actually drawing the chart. It can maybe be optimized a bit, but the problem will sooner or later reappear with even larger uploads.

[staticf0x]

Currently there's a hack in ds/lib/plot.py to reduce the number of plot points