put correlation coefficient on ggplot scatter plot after faceting - r

Question

I'm having issue to put correlation coefficient on my scatter plot after facet_wrap by another variable.
Below is the example I made using mtcars dataset for illustration purpose.
when I plot it out, both plot have the same correlation number. It seems the correlation coef is not calculated for each facet. I could not figure out a way to achieve that. Really appreciate it if anyone could kindly help with that...
library(ggplot2)
library(dplyr)
corr_eqn <- function(x,y, method='pearson', digits = 2) {
corr_coef <- round(cor.test(x, y, method=method)$estimate, digits = digits)
corr_pval <- tryCatch(format(cor.test(x,y, method=method)$p.value,
scientific=TRUE),
error=function(e) NA)
paste(method, 'r = ', corr_coef, ',', 'pval =', corr_pval)
}
sca.plot <- function (cor.coef=TRUE) {
df<- mtcars %>% filter(vs==1)
p<- df %>%
ggplot(aes(x=hp, y=mpg))+
geom_point()+
geom_smooth()+
facet_wrap(~cyl, ncol=3)
if (cor.coef) {
p<- p+geom_text(x=0.9*max(df$hp, na.rm=TRUE),
y=0.9*max(df$mpg, na.rm=TRUE),
label = corr_eqn(df[['hp']],df[['mpg']],
method='pearson'))
}
return (p)
}
sca.plot(cor.coef=TRUE)

Answer 1

Call facets through variable inputFacet, loop over this variable to calculate corr_enq and plot facets using variable name with get.
In shiny you'll probably have user input as input$facet here it's called inputFacet. We plot main plot getting this variable in facet_wrap(~ get(inputFacet), ncol = 3). Next we loop over all facet options with for(i in seq_along(resCor$facets)) and store result in rescore.
This should solve "correlation coef is not calculated for each facet" problem.
library(dplyr)
library(ggplot2)
inputFacet <- "cyl"
cor.coef = TRUE
df <- mtcars
p <- df %>%
ggplot(aes(hp, mpg))+
geom_point()+
geom_smooth()+
facet_wrap(~ get(inputFacet), ncol = 3)
if (cor.coef) {
resCor <- data.frame(facets = unique(mtcars[, inputFacet]))
for(i in seq_along(resCor$facets)) {
foo <- mtcars[mtcars[, inputFacet] == resCor$facets[i], ]
resCor$text[i] <- corr_eqn(foo$hp, foo$mpg)
}
colnames(resCor)[1] <- inputFacet
p <- p + geom_text(data = resCor,
aes(0.9 * max(df$hp, na.rm = TRUE),
0.9 * max(df$mpg, na.rm = TRUE),
label = text))
}
p