refs #7957. Fix works for Proper plot calc from slice.

The fix does not cover the preview plot. For some reason the Preview plot ends up with a 0,1,0,1 bounding box.. This will require some further investigation.
refs #7957. Remove custom CurveData

QwtPlotCurve can be used and does the right thing. Fix AutoScaleAxis usage.
Merge branch 'feature/7957_log_scale' into develop