I'm trying to create a custom curve fit from a data table such that it uses the previous y value from the previous x axis value in it's calculation.

Hi all, I'm trying to create a custom curve fit from a data table such that it uses the previous y value from the previous x axis value in it's calculation.  I'm trying to do this in a line chart.  

For example my Y axis is oil and my X axis is months, I want my curve fit to calculate a Y where X = n base on Y where X = N-1 if that makes sense.  I know this is probably not easy but I'm trying to mimic the part of arp's decline formula where it switches from hyperbolic to exponential decline.  Any help is appreciated.  It's very easy in excel, but I don't want to move it over there to do this.  

Thanks,

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