qchart scroll can freeze

The scroll function in qchart class sometimes can freeze.  It happens when the left position in the view port is a small number (i.e. 4) and scroll is called several times with a large negative number(i.e. --30 or -50).   Attached are some stack traces.

Another issue I found is that it is difficult to scroll to 0 position.  For example, if current left x is 5 and you call scroll with -5, ideally it should scroll the chart to the very beginning.  But it does not.

stacktrace_1

stacktrace_2

stacktrace_3

stacktrace_4

stacktrace_5