QwtPlotCurve Step CurveStyle incorrect when extending beyond QwtPlot edge

[philw]

We're finding that QwtPlotCurves with the "Steps" CurveStyle show a misleading value when extending beyond the visible area of the QwtPlot.

The following image shows three QwtPlots with the same series data, drawn with a line width of 2 pixels, shown with magnified insets:
.

• Top: Drawn with the "Lines" CurveStyle. Good.
• Middle: Drawn with the "Steps" CurveStyle. Shows problem.
• Bottom: Vertically zoomed out "Steps" view.

StepClip1.jpg

This image also available here:
http://cadswes2.colorado.edu/~philw/.../StepClip1.png

We're using Qwt 6.1.2 on Windows. We're seeing this with both Qt 4.8.5 and Qt 5.5.1.

We're hoping that we won't have to intervene at the application level -- detecting crossings of the plot edge and breaking the curve into multiple curves or segments.

Is this a known problem with the "Steps" CurveStyle?
Any ideas on what to do?
Thank you in advance.