I would like to do isotonic regression on bivariate data (that is to say, x with two columns). The function biviso of package Iso is very fast but requires a balanced grid. I would like to do isotonic regression on unbalanced grid. For example:
x1 <- runif(100) x2 <- runif(100) y <- x1*x2 + rnorm(100)
Is there an R package or custom R code that can do this?