PROC graphics1 (REAL CONST xmin, xmax, INT CONST steps,
               REAL PROC (REAL CONST) f):
   REAL VAR x, delta x :: (xmax - xmin) / real (steps - 1);
   move (graphics x limit DIV 3, graphics y limit DIV 2);
   put ("Computing...");
   determine minimal and maximal function values;
   page;
   draw the axes with annotation;
   draw the graphics.

determine minimal and maximal function values:
   x := xmin;
   REAL VAR ymin :: f (x), ymax :: f (x);
   WHILE x <= xmax
   REP
     x INCR delta x;
     IF ymax < f (x) THEN ymax := f (x)
     ELIF ymin > f (x) THEN ymin := f (x)
     FI
   ENDREP.

draw the axes with annotation:
   fix viewport and world;
   draw the axis x with annotation;
   draw the axis y with annotation.

fix viewport and world:
   INT CONST
      xvmin :: 10 * character width,
      xvmax :: graphics x limit - 10 * character width,
      yvmin :: graphics y limit - 2 * line height,
      yvmax :: 2 * line height;
   REAL VAR
      sx :: real (xvmax - xvmin) / (xmax - xmin),
      sy :: real (yvmax - yvmin) / (ymax - ymin),
      cx :: real (xvmin) - sx * xmin,
      cy :: real (yvmin) - sy * ymin.

draw the axis x with annotation:
   INT CONST y0 :: round (cy);
   move (xvmin, y0);
   draw (xvmax, y0);
   move (xvmin - 8 * character width,
         y0 - line height DIV 3);
   plot text (text (xmin, 7, 2));
   move (xvmax - 3 * character width DIV 2,
         y0 - line height DIV 3);
   plot text (text (xmax, 7, 2)).

draw the axis y with annotation:
   INT CONST x0 :: round (cx);
   move (x0, yvmin);
   draw (x0, yvmax);
   move (x0 - 5 * character width,
         yvmin + line height DIV 3);
   plot text (text (ymin, 7, 2));
   move (x0 - 5 * character width,
         yvmax - 4 * line height DIV 3);
   plot text (text (ymax, 7, 2)).

draw the graphics:
   x := xmin;
   move (round (x * sx + cx), round (f (x) * sy + cy));
   WHILE x <= xmax
   REP
     x INCR delta x;
     draw (round (x * sx + cx), round (f (x) * sy + cy))
   ENDREP.
ENDPROC graphics1;

program:
  enter graphics mode;
  graphics1 (0.0, 6.0 * pi, 200, REAL PROC (REAL CONST) tan);
  wait for confirmation (2 * graphics x limit DIV 3, 1);
  leave graphics mode.