plw3d with the altitude set to 90 degrees and different azimuths produces non-rectangular projections. For azimuths 0, 90, 180, etc. the result is a rectangle corresponding to the xy plane. But for other azimuths this projection is a parallelogram. See the attached plots which are from x08 but with the alt set to 90. I
would expect this to always give a rectangle regardless of the azimuth.
Notice also that the axis labels are not correct; those should degenerate into a straight line at alt 90.