GET the EQN from trendlines.... and more

Are you required to use the chart trendlines for this, because I would do all of this in the spreadsheet (except for the visualization). All of the available trendlines in the chart are linear (or can be made linear), so I would use the LINEST() function to get parameters for each trendline that I wanted to try (https://support.office.com/en-us/art...a-fa7abf772b6d ), measure "best fit" (not sure how you want to measure goodness of fit), then choose the desired trendline from that.

I'm not sure of all your requirements for this, but I would try to do most or all of the work in the spreadsheet. The chart would be useful for visualization of the result, if you want to visualize it at the end.

I'm not sure I understand your objection. The LINEST() function handles polynomials just fine. As explained in the help file I linked to previously (second to last bullet point before the Examples):

Originally Posted by Excel help

In addition to using LOGEST to calculate statistics for other regression types, you can use LINEST to calculate a range of other regression types by entering functions of the x and y variables as the x and y series for LINEST. For example, the following formula:

=LINEST(yvalues, xvalues^COLUMN($A:$C))

works when you have a single column of y-values and a single column of x-values to calculate the cubic (polynomial of order 3) approximation of the form:

y = m1*x + m2*x^2 + m3*x^3 + b

LINEST() can perform regressions for any linear function -- which extend way beyond straight line y=mx+b equation.

Is there some reason you cannot use the LINEST() function for these regressions? I still think LINEST() will be the easiest way.