Polynomial Regression

The polynomial regression can be generated by the RDOUBLE formula below.

The function in the example below is for a polynomial regression of the third degree.

If you require a function of a different degree, simply edit the 3 in the poly function to whichever degree you need.

To keep this approach generalized, I did not convert the values to a time series (ts). Instead, I used the rank function to convert each x-axis element to a vector of 1-n by using the RANK function.

rdouble(FALSE, "
 x <- args[[2]];
 y <- args[[1]];
 model <- lm(y ~ poly(x, 3));
 predict(model);
", [Total Cost], rank(max([Months in Date]), "ASC", "1223"))