Hi Werner,
Many thanks for your help with this. I have had another look at your Mathcad spreadsheet - and have taken some screen captures - below. However, what I have done is change the "limits" on the Y axis - and secondary Y axis as you will see - to see what is happening at values below zero.
You are correct - I do want the graph plot to "stop" - when the Y value dips below zero - because in reality the Y value can't have a value below zero. However, what is puzzling me - is why can't the plot "stop" when the Y value dips below zero?
Looking at your graphs below - even with "rubbish" removed - the plot carrys on horizontally - when it really should stop.
Look forward to your thoughts on this.
Gordon.
