Labeling a ggplot with a mix of variables and expressions - r

Question

I'm trying to make a labeled scatterplot in ggplot and the specifics of the labels are causing me fits. Basically, among other things, I want to annotate my facet_wrapped 2-panel ggplot with the R^2 and Mean Bias. Notably, I want to label the mean bias with the appropriate units.
A simple version of my data might look as follows:
library(tidyverse)
Demo_Df <- tibble(Modeled = rnorm(50,0,1), Observed = rnorm(50, 0.5, 1),
Scheme = c(rep("Scheme1", 25), rep("Scheme2", 25)))
Demo_Annotation <- tibble(r.squared = c(0.589, 0.573), Mean_Bias = c(-2.038, -1.049), Scheme = c("Scheme1", "Scheme2"))
Demo_Scatter <- Demo_Df %>%
ggplot(aes(x = Observed, y = Modeled, color = Scheme)) +
geom_point(size = 1.5) +
facet_wrap(~Scheme) +
theme_tufte() +
xlab(expression(paste("Observed Aerosol (", mu, "g m" ^ "-3", ")"), sep = "")) +
ylab(expression(paste("Modeled Aerosol (", mu, "g m" ^ "-3", ")"), sep = "")) +
ylim(-3, 4) +
theme(legend.position = "none")
Demo_Labeled <- Demo_Scatter +
geom_text(data = Demo_Annotation, aes(-2, 3,
label = paste(
"R2 = ", sprintf("%.2f", signif(r.squared, 3)), "\n",
"Mean Bias = ", sprintf("%.2f", signif(Mean_Bias, 3))
)),
size = 5, hjust = 0, color = "black")
This produces almost the right figure, but I would like the R2 to have a superscript 2 and I need to add micrograms per cubic meter (ug/m3) to the end of the "Mean Bias = " label, as it is on the x and y-axes.
To date, I've completely failed at this. I cannot find a solution that supports multiple lines, facet_wrap, variable inputs, AND expressions. There has to be a way to do this. Please help me, tidyverse gods!

Answer 1

One option to achieve your desired result is to add you multiple lines via multiple geom_text layers. To parse the labels as math notation add parse=TRUE to geom_text. Finally I added the labels to you annotations df where I made use of ?plotmath for the math notation.
library(tidyverse)
library(ggthemes)
Demo_Annotation <- Demo_Annotation %>%
mutate(r.squared = paste0("R^{2} == ", sprintf("%.2f", signif(r.squared, 3))),
Mean_Bias = paste0("Mean~Bias == ", sprintf("%.2f", signif(Mean_Bias, 3)), "~mu*g~m^{-3}"))
Demo_Scatter +
geom_text(data = Demo_Annotation, aes(x = -2, y = 4, label = r.squared),
size = 5, hjust = 0, color = "black", parse = TRUE, family = "serif") +
geom_text(data = Demo_Annotation, aes(x = -2, y = 3.5, label = Mean_Bias),
size = 5, hjust = 0, color = "black", parse = TRUE, family = "serif")
DATA
set.seed(42)
Demo_Df <- tibble(Modeled = rnorm(50,0,1), Observed = rnorm(50, 0.5, 1),
Scheme = c(rep("Scheme1", 25), rep("Scheme2", 25)))