How to write an equation with a variable in legend?

Question

I am trying to write an equation like "R^2=0.00575" in the legend, and the number 0.00575 can be embedded in the legend automatically. Here is an example.

set.seed(100)
x=rnorm(100)
y=1:100
fit=lm(y~x)
R_squared=format(summary(fit)$r.squared,digits = 3)
plot(x,y,type="l")
legend("topleft",legend =expression(R^{2}~"="~R_squared),bty = "n")

As the figure shows, the variable "R_squared" is not embedded in the equation. Is there any solution? Thanks.

Answer 1

For this task I think it is best to do parse(text=sprintf(...)). You can code the R language syntax into the string literal to be parsed into an R expression using parse(), and use sprintf() format specifications to embed any numeric or string values that are stored in variables into the expression.

set.seed(100L);
x <- rnorm(100L);
y <- 1:100;
fit <- lm(y~x);
R_squared <- format(summary(fit)$r.squared,digits=3L);
plot(x,y,type='l');
legend('topleft',legend=parse(text=sprintf('paste(R^2,\' = %s\')',R_squared)),bty='n');

An alternative syntax that leverages the fact that == is plotted as a single equal sign:

legend('topleft',legend=parse(text=sprintf('R^2 == %s',R_squared)),bty='n');

See the plotmath documentation.

Answer 2

You can also use bquote:

set.seed(100L);
x <- rnorm(100L);
y <- 1:100;
fit <- lm(y~x);
R_squared <- format(summary(fit)$r.squared,digits=3L);
plot(x,y,type='l');
legend('topleft',legend=bquote(R^{2} ~ "=" ~ .(R_squared)),bty='n');

More information on partial substitution of expressions with bquote can be found here, which defines the function as:

An analogue of the LISP backquote macro. bquote quotes its argument except that terms wrapped in .() are evaluated in the specified where environment.