The PLplot Plotting Library: Programmer's Reference Manual  

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plgradient
: Draw linear gradient inside polygon
Draw a linear gradient using colour map 1 inside the polygon
defined by the n
points
(x
[i],
y
[i]). Interpretation of the polygon
is the same as for plfill
. The polygon coordinates and the gradient
angle are all expressed in world coordinates. The angle from the
x
axis for both the rotated
coordinate system and the gradient vector is specified by
angle
. The magnitude of the
gradient vector is the difference between the maximum and minimum
values of x
for the vertices
in the rotated coordinate system. The origin of the gradient vector
can be interpreted as being anywhere on the line corresponding to the
minimum x
value for the
vertices in the rotated coordinate system. The distance along the gradient
vector is linearly transformed to the independent variable of colour
map 1 which ranges from 0. at the tail of the gradient vector to 1. at
the head of the gradient vector. What is drawn is the RGBA colour
corresponding to the independent variable of colour map 1. For more
information about colour map 1 (see the Section called Color Map1 in Chapter 3).

Redacted form: plgradient(x,y,angle)
This function is used in examples 25,30.