graph: yet one tooltip tweaking

graph: yet one tooltip tweaking

Don't use a percentage of the data value as the check point, that
makes it increasingly difficult to pin point a tooltip for a graph
that is near the bottom end of the range. Instead use a fixed
percentage of the first tickmark, that makes it equally fair
across the range.

Also switch to using the X axis as the prio tree index, since that
should be increasing nicely all the time.

Signed-off-by: Jens Axboe <axboe@kernel.dk>