The simplest way to "invert" the trendline is to switch the dependent and independent variables. In other words, create a series with X1:X100 for the y-axis and Y1:Y100 for the x-axis. Choose the simplest trendline that has a reasonably good RSQ, albeit maybe not the best RSQ. The trendline will always read y=...x... Just "translate" x to mean Y1:Y100 and y to mean X1:X100.
If you do choose a high-degree polynomial to fit your data, beware that it is probably only useful for interpolating data points. Such formulas tend to "blow up" when you try to extrapolate them.
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