136 The &97 in line 240 produces white graphic dots. Other values will give other colours. For example &91 would draw a red graph. Two improvements could be made; first we could test for illegal values of X% and Y% with 320 IF X% 75 OR Y% 56 THEN ENDPROC and secondly remember the position of the cursor before we entered the procedure and restore the cursor to that position at the end of the procedure. Notice that you do not need to reference actual memory coordinates and this is vital if your programs are to work via the Tube.
You may not think it is important now but you will find that it is advisable to write programs using the machine code calls provided and not to get into the habit of addressing the memory directly.