How do you pull an equation off of a scatterplot with smooth lines in Excel?

Zach Hill 1 Reputation point

I need the equation of a "smooth line" in a scatterplot that looks almost logarithmic. I need the equation so that I can determine the intercept of another line crossing it, without approximating. It's a stress-strain curve for anyone who knows what that is.

The trendlines I'm offered (no matter which type or order/period of the function) are nowhere close to fitting the actual curve. The logarithmic trendline returns an error and won't display itself (possibly because the curve dives down and away from its asymptote).

I played around with making my own functions and plugging the variables into the built-in Excel Solver which gave me the same results as the shoddy trendlines.

I'm now wondering how Excel knows how to plot the smooth line through these data points. Is there a way to get the equations of these splines (or whatever they may be) off of the graph? Before you answer, I know about the decimal place workaround thing for trendlines; I'm out to 30 decimal points.

Please help!

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  1. Herbert Seidenberg 1,191 Reputation points

    Excel 365
    Fit a curve to some stress-strain test points.
    External math program and degree desirable.