Synchronizing Y-axis scaling when zoomed with QwtPlotZoomer for two curves

[bob2oneil]

I have noticed an interesting behavior when using the
QwtPlotZoomer tool on a plot which consists of two
curves. The curves, as shown in the attached screenshot,
are a cosine and sin wave. Autoscaling is enabled for
both Y-axis. The values range from 1 to -1 for each curve.

When the zoom rectangle is used to zoom in on the curve,
the Y-axis do not scale in the same manner, which looses
the fidelity and alignment between them on the plot.
This is shown as ChartZoom.jpg as an attachment.

How can I get similar behavior from the two axis when
using the QwtPlotZoomer class when essentially the same
autorange value is defined for both Y-axis (here the
range is -1 to 1)?