I have the exterior shape of a nozzle given by S(x). Now what I want to
do is first reflect it with respect to the y=0 axis (easily achieved by
-S(x) right?) and then map each x node's value of an array (say density)
to a color that would fill the vertical space between S(x) and -S(x). In
other words I need something like the matplotlib.pyplot.fill_between
method using x, S(x) and -S(x) but where the fill color would be
relative to a third array (say density again) at each point x.