Much easier to wrap one's head around it if it is expressed more
directly.

This is not mathematically the same as the previous version involved
more rounding than necessary because of the way the intervalFactor was
handled. I'd argue the new version is better because it gets closer to
the 11,000 points limit and thus approaches better what the user
wanted within the limits of Prometheus.

Note that in practice, the 11,000 points limit should never be of
relevance. (Even a 4k screen doesn't have 11k points on the x axis.)

Signed-off-by: beorn7 <beorn@grafana.com>