Previously there would sometimes be a long line of equal time labels
if you selected a small range from the middle of a long trace. This
patch increases the accuracy of the labels based on selected range,
not on start time.

Task-number: QTCREATORBUG-10865

Change-Id: I482da242c5e9f0117611f100888e80e23ad91b4c
Reviewed-by: Kai Koehne <kai.koehne@digia.com>