Hi Casey, Thank you. Yes I did and found nothing.
I can do the histogram and this is better than a single, often jagged line. I did not think it would be difficult to do the colour change at the zero (or other selected) line.
I am attaching the seeming successful graph that GraHa on PRC put up but has not got back to me about. (I refer to it in my reply to jlz above).
If you or any other can work out how he did it, in case he does not get back to me, then I would be grateful.