Extension of ggplot2 to work friendly with lm object

ggplot2_lm.R

library(ggplot2)
require(ggplot2)

# usage
#
# stat_r2(lm_obj, x, y, adjusted = FALSE, digits=NULL, ...)
#   lm_obj      lm object given by lm()
#   x, y        positional coordinate of r2 value
#   adjusted    use adjusted R2 instead
#   digits      see format()
#   ...         other arguments passed to geom_text()
#
# geom_lm_line(lm_obj, x = NULL, ...)
#   lm_obj      lm object given by lm()
#   x           string indicating which variable to draw; default NULL to use first independent variable
#   ...         other arguments passed to geom_abline()

stat_r2 <- function(lm_obj, x, y, adjusted = FALSE, digits=NULL, ...){
  stopifnot(is.numeric(x), is.numeric(y))
  value <- ifelse(adjusted, summary(lm_obj)$adj.r.squared, summary(lm_obj)$r.squared)
  stopifnot(is.numeric(value))

  r2 <- as.character(as.expression(paste0(ifelse(adjusted,
                                                 'plain(Adjusted)~{R^2} == ', 'R^2 == '), 
                                          as.character(format(value, digits=digits)))))
  dd <- data.frame(x=x, y=y, r2=r2)
  return(geom_text(data=dd, mapping=aes(x=x, y=y, label=r2, shape=NULL), parse=T, ...))
}

geom_lm_line <- function(lm_obj, x=NULL, ...){
  co <- lm_obj$coefficients
  stopifnot(is.numeric(co[[1]]))
  return(geom_abline(intercept=co[[1]], slope=ifelse(is.null(x), co[[2]], co[[x]]), ...))
}

geom_lm_line() is very handy