Label ggplot facet plots with facet group name and R^2

facetR2.R

# From https://stackoverflow.com/questions/17022553/adding-r2-on-graph-with-facets
# Function for calculating R^2
# Adjust the lm call as required
lm_eqn = function(df){
  m = lm(min_t ~ max_t, df);
  eq <- substitute(r2, 
             list(r2 = format(summary(m)$r.squared, digits = 3)))
  as.character(as.expression(eq));                 
}

# Create dataframe containing the R^2, species and a facet label
# Adjust the column names and paste details, as required
eqns <- by(pdat, pdat$code, lm_eqn)
df2 <- data.frame(eq = unclass(eqns), species = names(eqns))
df2$lab = paste(df2$species, "R^2 =", df2$eq, sep=" ")

# Make a labeling function that will refer to your data frame of labels
r2_labeller <- function(variable,value){
  return(df2$lab)
}

# Add the labeller function to facet_wrap
facet_wrap( ~ code,  ncol=2, scales = "free", labeller = r2_labeller)