This gives the color range from red through violet corresponding to values 0 to 1. If you happen to be stuck in a backwards, primitive country, tough luck. Bite the bullet. Minor unobvious things have prevented people from getting graphs. Most problems occur when badly formed xyplop files are used, and the program has no way to tell what the difficulty is.
More checks have been put it, so the program can detect most oddly formed xylop and xyin files.
Check your xyplop carefully. Variable Y was not initialized before this use. Variable X was not initialized before this use.