The R Book

Axis Labels Containing Subscripts and Superscripts

The default xlab and ylab do not allow subscripts like r2 or superscripts or xi. For these you need to master the expression function. In R, the operator for superscripts is ‘hat’ (or, more correctly, ‘caret’) so ‘r squared’ is written r^2. Likewise subscripts in R involve square brackets [] so x_i is written x[i]. Suppose that we want r² to be the label on the y axis and x_i to be the label on the x axis. The expression function turns R code into text like this:

plot(1:10,1:10, ylab=expression(r^2), xlab=expression(x[i]),type="n")