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Astonished to find there is still no way to add a 45 degree reference line, without hacking. This should be built in. Actually, what would be most useful would be the ability to add any polynomial curve by using a formula - this would be useful eg where a theory predicts most data points will lie along a curve .... such and such - so here's the theory, and here's the reality. This is the natural extension of the trend lines, ie predicted trend lines. You have the code for this already. So my 45 degree line is just the very simple x=y curve!