Change the window in world coordinate space that is to be mapped on
to the viewport, and simultaneously adjust the viewport so that the
world-coordinate scales are equal in x and y. The new viewport is
the largest one that can fit within the previously set viewport
while retaining the required aspect ratio.
Arguments
X1 (input) : the x-coordinate of the bottom left corner
of the viewport.
X2 (input) : the x-coordinate of the top right corner
of the viewport (note X2 may be less than X1).
Y1 (input) : the y-coordinate of the bottom left corner
of the viewport.
Y2 (input) : the y-coordinate of the top right corner of the
viewport (note Y2 may be less than Y1).