Draws a 3-dimensional graph
  Michael Castleman, mlc@sri.kbt.com

Using GRAPH3
------------
GRAPH3 draws a 3-dimensional graph of the plane AX+BY+CZ=D. To use it, run
GRAPH3. You will then be prompted for A,B,C, and D. Input them. Note that
you may use expressions if you need to. After you finish, the X, Y, and Z
intercepts will be displayed. After a brief pause, the graph will be drawn.
You do *not* need to press [ENTER] to display the graph. The three axes are
drawn, with the dotted portions representing the parts of the axes that
cannot be seen. The intercepts are plotted, and then will be connected with
three lines. Note that GRAPH3 will _not_ work if either A, B, or C is zero.

Using GRAPH3N
-------------
GRAPH3N resets the graph display after using GRAPH3. To use it, simply run
GRAPH3N. After a very short time, you will be returned to the home screen.
Press [GRAPH] to redraw any 2-dimensional functions (if any) you may have
entered.

How does GRAPH3 work?
---------------------
After getting the values for A,B,C, and D, the intercepts must be calculated.
This can be done by Xintercept=D/A, Yintercept=D/B, and Zintercept=D/C. These
values are stored in a 3x3 matrix with the X, Y, and Z coordinates for each
point. Then, these are converted to values on a 2-dimensional coordinate
system. The Y and Z axes correspond to the X and Y axes, and the X axis is
\sqrt\.5A\^2\ along both axes, where A is the X coordinate. If X is positive,
the opposite of the above value must be found. Then, using a stat plot and
lists L1 and L2, the 3 intercepts are graphed. The lines are drawn using
Line( commands for the solid and For( commands for the broken parts. Then,
connecting lines are drawn between the 3 intercepts.

Memory Used
-----------
        Type Name                       Amt
        ---- -------------------------- ---
        prgm GRAPH3                     570
             GRPAH3N                     23
        list L1, L2             2 * 35 = 70
        mtrx [A]                         89
        real A,B,C,D,E,Y        6 * 15 = 90
                                        ---
                                        842
All non-program items may be safely deleted.