void plane3d (plane, position, hide) D3PLT.C Level 2
int plane = 1 for the x-y plane. The x and y axes will be used to determine the scaling.
= 2 for the x-z plane. The x coordinates will scale to the x axes; the y axis will scale to the z axis.
 = 3 for the y-z plane. the x coordinates will scale to the y axes; the y axis will 
scale to the z axis.
float position The position, in user units, of the perpendicular axis at the intersection of the specified plane.
int hide = 0 Hidden line removal is disabled.
= 1 Hidden line removal is enabled; lines that fall between horizons will not be drawn. Any lines drawn will update the 3-D horizons.
= 2 Any lines that go between the upper and lower horizons will appear as dotted lines. The horizons will not be updated.

Hidden line removal works by establishing upper and lower horizons for each pixel column. Surfaces are plotted starting from the near corner, and the horizons are updated so that for each column the upper horizon is set to the highest point of the surface and the lower horizon is set to the lowest. When the surface is finished, the horizons define the outline of the surface.

After a call to plane3d() all line drawing will be scaled to the 3-D workbox established by the most recent call to graf3d(). 2-D plots should be generated normally except that calls to scales() and graf() will be ignored and are not necessary.

Call plane3d(0, 0.0, 0) to reset the program for normal 2-D plotting. This reset should be done before endplot()
otherwise, the next plot, a zoom, a replay, or any printouts will be projected.