Philip M It is a curve! Look at the scale on the 'y' axis -- it's logarithmic. But I suspect you are winding me up! |[:-)No no no. I never do that.. I never looked close enough, and I always suspect graphs..But a straight line is still straight.. It isn't a curve. ??? Philip, in you comment to Allen D, you remark on my 'nit-picking' of your posts. I only do this where the error gives rise to ambiguity -- I want to know what you mean. I've found it safer to assume that people mean what they say, rather than assessing their statement to mean something which they did not say. Paul D You are correct of course.. But I always expect the reader to be a mindreader as well. It helps. Phil.