plgdiplt: Get parameters that define current plot-space window

Get relative minima and maxima that define current plot-space window. If plsdiplt has not been called the default values pointed to by p_xmin, p_ymin, p_xmax, and p_ymax will be 0., 0., 1., and 1.

p_xmin (PLFLT *, output) Pointer to relative minimum in x. p_ymin (PLFLT *, output) Pointer to relative minimum in y. p_xmax (PLFLT *, output) Pointer to relative maximum in x. p_ymax (PLFLT *, output) Pointer to relative maximum in y.