# Offsetting a curve

From support e-mail:

Q: I have created a log-prob curve and have fitted an equation to it that fits very nicely. Now I would like to create a parallel curve through another known data point How would I do this?

A: Nothing to it. You want your new curve to have the same shape (on this scale) as the original curve but offset by some value. So in your case the logarithms of the Y values in the new curve should all be proportional to the logarithms of the Y values in the original curve. First copy your curve fit to the Clipboard using Edit>Copy>Data Values and then paste using Edit>Paste so that you have a copy of that curve and can preserve the original.

It's good in your case that you have a known offset between two points with the same X values. Otherwise this would take a smidgen more work. But basically, you want the same shape curve to pass through the point X=4%, Y=553, where the original passes through X=4%, Y=2697. So the new curve should be something like Y'=10^(log10Y + log10553 - log102697. Remembering a bit of high school algebra, this reduces to Y'=Y*553/2697 (though either form will work). Use the Operate on Y command on the Edit menu on your pasted curve with Y=Y*553/2697 to get:

... which mimics the original curve fit with an offset in Y.

This page contains a single entry by David Hyde published on June 18, 2009 6:47 PM.

Finding 0.2% offset strain was the previous entry in this blog.

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