Use a variable within a plotmath expression

Use bquote(). Here is an example with dummy data:

set.seed(1)
DF <- data.frame(A = rnorm(100), B = rnorm(100))
mod <- lm(B ~ A, data = DF)
R2Val<-signif(summary(mod)$r.squared[1], 2)

The parts of the expression wrapped in .() get evaluated in the environment, i.e. the value of R2Val is substituted.

plot(B ~ A, data = DF)
text(1.5, 2, labels = bquote(R^2 == .(R2Val)), adj = 0, cex = 0.85)

Another potential solution is substitute, which works similarly:

plot(B ~ A, data = DF)
text(1.5, 2, labels = substitute(R^2 == A, list(A = R2Val)), adj = 0, cex = 0.85)

To combine substitute() and paste(), I use the following code;

waic <-1;
chisquare <-2;
plot(x=1:2,y=1:2, 
     main =  substitute(paste(
    "chi^2 goodness of fit ", 
       chi^2*(D/theta) , "=",  chisquare ,
       ".  WAIC =",waic),


list(chisquare=chisquare,waic=waic)  
)
)

The result is the following;