void usertopix (x_in, y_in, z_in, x_inch, y_inch) SVC.C Level 3
float x_in, y_in, z_in The input coordinates in user units. The z_in value is ignored in 2-D plots.
int *x_pix, *y_pix The returned pair in GraphiC pixel space.
usertopix() takes an x, y pair (2-D plots) or an x, y, z triplet (3-D plots) in your user units and returns the pixel 
values in the 2-D GraphiC pixel space. The function works for linear, log, and polar plots. For the latter, x is the input 
radial coordinate, and y is the input angular coordinate. The returned coordinates are in x,y space.

If the plot is rotated, the returned x pixel coordinate increases downwards, and the y pixel coordinate increases to the right (on the screen). Thus, the pixel coordinates always assume that the plot is upright and unrotated.