Adding R^2 on graph with facets

Question

I am plotting geom_points on multiple facets and would like to annotate R^2 on each facet (preferably on the facet_label rather on the graph.) I have found some code here wh

Answer 1

The following seems to work

  lm_eqn = function(df){
    m = lm(ln_x ~ ln_y, data=df)
    eq <- substitute(~~R^2~"="~r2, 
                      list(r2 = format(summary(m)$r.squared, digits = 3)))
    c(eq = as.character(as.expression(eq)));                 
 }

Create new data frame containing R^2 for each roi_size

labeldata <- ddply(df,.(roi_size),lm_eqn )

and geom_text becomes

geom_text(data=labeldata,aes(x=1.5,y=2.2,label=eq,family="serif"), color='blue',  parse=TRUE)

Answer 2

You can create a new data frame containing the equations for the levels of roi_size. Here, by is used:

eqns <- by(df, df$roi_size, lm_eqn)
df2 <- data.frame(eq = unclass(eqns), roi_size = as.numeric(names(eqns)))

Now, this data frame can be used for the geom_text function:

geom_text(data = df2, aes(x = 1.5, y = 2.2, label = eq, family = "serif"), 
          color = 'blue',  parse = TRUE)

Answer 3

Here is a complete solution including insertion of the R^2 value into the facet labels, otherwise built upon the solution from Sven Hohenstein.

First, change the function for getting the R^2 values so that it only grabs the number without any extra text

lm_eqn = function(df){
  m = lm(ln_x ~ ln_y, df);
  eq <- substitute(r2, 
                   list(r2 = format(summary(m)$r.squared, digits = 3)))
  as.character(as.expression(eq));                 
}

put equations for each roi_size into a dataframe (as with Sven's solution),

eqns <- by(df, df$roi_size, lm_eqn)
df2 <- data.frame(eq = unclass(eqns), roi_size = as.numeric(names(eqns)))

but then concatenate them with the roi_size in a new column

df2$lab = paste("roi_size =", df2$roi_size, "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)
}

then plot, using the labeling function while calling facet_wrap

ggplot(df, aes(x=ln_x, y=ln_y)) +
  geom_point(shape=19, aes(colour=factor(depth))) + 
  geom_smooth(method="lm") + 
  facet_wrap(~roi_size, labeller = r2_labeller) + 
  scale_color_discrete("depth (mm)") +
  labs(y=expression(ln(frac(C[low]^air,C[low]^depth))),
           x=expression(ln(frac(C[low]^depth,C[high]^depth))) ) +
  theme(axis.title.x = element_text(colour='blue', size=16, hjust=0.9)) + 
  theme(axis.title.y = element_text(colour='blue', size=16, angle=0))