Graph doesn't go back as far as the first recorded errors

The graph currently starts 90 days after the first recorded errors, because that's when we start being able to divide by the number of reporting machines that were active over the past 90 days.

We should be able to extrapolate the curve of active machines back to a couple of weeks after the Ubuntu 12.04 release. (If we tried to go beyond that we'd be messed up by the huge influx of 12.04 users after the 12.04 release.)