Syntax: SHADE INTEGER pexpr pexpr
or      SHADE HISTOGRAM INTEGER pexpr pexpr

   (`Pexpr' is the name of a vector, or an expression in parentheses,
e.g. SHADE 1000 x (sqrt(y)))

   Shade `inside' a curve defined by the expressions. The shading is
rotated through the current value of ANGLE, and lines are spaced
INTEGER apart (screen coordinates, so the full screen is 32768
across), 500 is about right. If INTEGER is 0, the lines will be drawn
close enough to fill the area, but this is liable to be rather slow.
The meaning of `inside' is that as the shading is done, from left to
right taking the value of ANGLE into account, lines are drawn from
every odd to every even crossing of the curve. The curve is considered
as being closed by joining the first to the last point. If a shading
line just touches the curve the algorithm may be confused, change
INTEGER slightly.  Sometimes joining the ends of the curve may not be
what you want, try using CONCAT to add points on the end yourself. For
example, `SET x=0,10 SET y=x**2 LIMITS x y SHADE 1000 x y' looks like
a new moon, but `SHADE 1000 (x CONCAT 10) (y concat -1e10)' shades
beneath the curve, for ANGLE 0 that is.

   SHADE HISTOGRAM is similar, but it shades the histogram that would
be drawn by HISTOGRAM from the same set of points. In this case the
area to be shaded lies between the histogram and the line y=0. If this
offends you, offset the whole graph, and lie about the axes.