R: nls not picking up additional arguments when used in custom function in geom_smooth

Question

This is a question that relates to my earlier question geom_smooth with facet_grid and different fitting functions. In that question, I was trying to use a different fitting

Answer 1

Here's the solution, which greatly benefited from this post. I don't know why the previous version didn't work, but this seems to work fine.

# Load library
library(ggplot2)

# Load data
data(mtcars)

# Smoothing function with different behaviour depending on the panel
custom.smooth <- function(formula, data,...){
  smooth.call <- match.call()

  if(as.numeric(unique(data$PANEL)) == 6) {
    # Nonlinear regression
    smooth.call[[1]] <- quote(nls)
    # Specify formula
    smooth.call$formula <- as.formula("y ~ a * x ^ b")
    # Add initial parameters
    smooth.call$start <- c(a = 300, b = -0.5)
  }else{
    # Linear regression
    smooth.call[[1]] <- quote(lm)
  }

  # Perform fit
  eval.parent(smooth.call)
}

# Plot data with custom fitting function
p <- ggplot(mtcars,aes(x = disp, y = mpg)) + geom_point() + facet_grid(gear ~ am)
p <- p + geom_smooth(method = "custom.smooth", se = FALSE)
print(p)