Here's an implementation of the bresenham line drawing algorithm for
a Heath-19 terminal (IIRC.)  This is very old and part of my "learn C"
period, so it's probably not ideal!

BillW

                ---------------


/* Test routine for Bresenham line drawing code */
/* this program is an aid to debugging HPGRAPH.ASM */

#include <stdio.h>

main()
{
 int x1,y1, x2,y2, ttystat, i,ii;
 int line1[4], delta1[4],limits[4];
 limits[1]=limits[3]=79;
 limits[2]=limits[4]=23;

 while (1) {
  for (i=1;i<=4;i++) line1[i]=0;
  delta1[1] = nread();
  delta1[2] = nread();
  delta1[3] = nread();
  delta1[4] = nread();
  ttystat= binon();
  printf ("\033H\033J");
  for (i=1; i<=99; i++) {
        for (ii=1; ii<=4; ii++) {
                line1[ii] += delta1[ii];
                if (line1[ii] < 0) { line1[ii]=0; delta1[ii] = -delta1[ii]; };
                if (line1[ii] > limits[ii]) {line1[ii]=limits[ii];
                                           delta1[ii]= -delta1[ii]; };
        };
        DrawLine(line1[1],line1[2],line1[3],line1[4],'*');
        DrawLine(line1[1],line1[2],line1[3],line1[4],' ');
  }
  binoff(ttystat);
 }
}

int nread()
{
 int c,n;
 n = 0;
 while ((c=getchar()) >= '0') n= n*10 + (c - '0');
 return n;
}

xchg(i,j) int i,j; { int temp; temp=i; i=j; j=temp; }

int binon() { int ablock[5],t;  ablock[1]=0101;
        jsys(0107,ablock); /* rfmod */
        t= ablock[2];
        ablock[2]=0;
        jsys(0110,ablock); /* sfmod */
        return t;
}
binoff(t) int t; {int ablock[5]; ablock[1]=0101;
        ablock[2]=t;
        jsys(0110,ablock); /* sfmod */
}

DrawLine (xin1,yin1, xin2,yin2,pchr)
int xin1,yin1, xin2,yin2, pchr;
{
#define DXgtDY 0
#define DYgtDX 1

 int x, y, x1, y1, dx,dy, e, e1, e2, foo, i;

 if (xin1>xin2)
        {x=xin2; x1=xin1; y=yin2; y1=yin1;} /* swap so that dx >0 */
  else  {x=xin1; x1=xin2; y=yin1; y1=yin2;};

 dx= x1-x;      /* dx is always > 0 */
 dy= y1-y;

 foo=DXgtDY;

 if (dy > 0)  {
  if (dy > dx)  foo= DYgtDX;
 } else {
  if (-dy > dx)  foo= DYgtDX;
 }

 printf ("\033Y%c%c", y+32,x+32);  /* initial position */
 switch (foo) {

  case DXgtDY :
        e = 2*dy - dx;
        e1 = 2*dy;
        e2 = 2*dy - 2*dx;
        if ( dy > 0) {
          for (i=1; i<=dx; i++) {
            putchar(pchr);              /* plot x,y */
            if (e > 0)   { e-= 2*dx; y+=1; printf("\033B"); }
            e+= 2*dy;
            x += 1;
          };
        } else {
          for (i=1; i<=dx; i++) {
            putchar(pchr);  /* plot(x,y) */
            if (e < 0)   { e+= 2*dx; y-=1; printf("\033A"); };
            e += 2*dy;
            x += 1;
          };
        }; break;

  case DYgtDX:
        i=dy; dy=dx; dx=i;
        e = 2*dy - dx;
        e1 = 2*dy;
        e2 = 2*dy - 2*dx;
        if ( dx > 0) {
          for (i=1; i<=dx; i++) {
            putchar(pchr);  /* plot(x,y) */
            if (e > 0)   { e-= 2*dx; x+=1; } else {putchar(8);};
            e += 2*dy;
            y += 1; printf("\033B");
          };
        } else {
          dx = -dx;     /* make a positive count of points */
          for (i=1; i<=dx; i++) {
            putchar(pchr);  /* plot(x,y) */
            if (e > 0)   { e+= 2*dx; x+=1; } else {putchar(8);};
            e += 2*dy;
            y -= 1; printf("\033A");
          };
        }; break;
  };
 printf("\n"); /* print buffer */
 }