plalt is used to set the viewing altitude for three dimensional
plots. This is expressed in degrees above the XY plane.
This is not an built-in function. This function
is normally loaded on start-up from the plot.r file in the
standard rlib directory. Use of the -r option, incorrectly
setting the RLAB_LIB_DIR environmental variable, or modifying
plot.r may make this function unavailable.