[R] ggplot: combining geom's in function

[sbihorel]

Hi,

Next adventure into my journey from lattice to ggplot: I would like to 
create a custom generic function that combines multiple existing geom's  
in order to reproduce what the lattice panel.xyplot function does based 
on the type argument (ie, plotting points only for type='p', plotting 
lines for type 'l', etc).

My current naive attempt is:

library(lattice)
library(ggplot2)

geom_xyplot <- function (mapping = NULL, data = NULL, stat = "identity",
                          position = "identity", na.rm = FALSE, type = 
'p', ...) {

   if (any(type=='p')){
     geom_point(mapping = mapping, data = data, stat = stat,
                position = position, na.rm = na.rm, ...)
   }
   if (any(type=='l')){
     geom_path(mapping = mapping, data = data, stat = stat,
               position = position, na.rm = na.rm, ...)
   }
   if (any(type%in%c('b','o'))){
     geom_point(mapping = mapping, data = data, stat = stat,
                position = position, na.rm = na.rm, ...) +
       geom_path(mapping = mapping, data = data, stat = stat,
                 position = position, na.rm = na.rm, ...)
   }
}

data <- data.frame(x = rep(1:4, each = 25),
                    y = rep(1:25, times = 4),
                    g = rep(1:4, each = 25))
data$x <- data$x + 0.005*(data$y)^2-0.1*data$y+1

ggplot(data2, aes(x, y, group = g, colour = factor(g))) + 
geom_xyplot(type = 'l')

I get:
 > Error: No layers in plot