[perf] Fix clicking on the minigraph in cluster summary.
[perf] Fix clicking on the minigraph in cluster summary.

Using 'index' doesn't take into account missing data points, while using pt[0], i.e.
the x-value, gives the right header element.

Bug: skia:
Change-Id: Ie39081c0c82106f3a27c36fcad7ad23795221978
Reviewed-on: https://skia-review.googlesource.com/38900
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot