How to add nice formated anotations to a R base graph using expression and the value of a variable?

Question

Say, I have a variable rv which has some numerical value. Now, I want to plot the value of this variable on a base plot but preceded by a nicely formatted symbol e.g., r subscript m, using expression. To write on the plot I use mtext.
However, what I get is either the value of the variable, but no nicely formatted symbol (left annotation), or a nicely formatted symbol, but not the value of the variable, but the variable name...

I tried to play around with eval, but didn't get what I wanted. Here is my code:

plot(1:10, rep(10,10), ylim=c(0,12))
rv <- 0.43

#left annotation:
mtext(paste(expression(italic(r[M])), " = ", rv), side = 1, line = -1.5, adj = 0.1)
#right annotation:
mtext(expression(paste(italic(r[M]), " = ", rv)), side = 1, line = -1.5, adj = 0.9)

This is the result:

How do i get both, nice format and value of the variable? Thanks.

btw: I know that I can get it, if I use two times mtext and play around with adj and stuff. But I would really like to get it in one call or without playing around with the position of two annotations.

Answer 1

Just combine what you have and plot two pieces, joined by using adj:

R> plot(1:10, rep(10,10), ylim=c(0,12))
R> text(2,12, expression(paste(italic(r[M]))), adj=1)
R> text(2,12, paste("=", rv), adj=0)

Answer 2

The bquote function will create an expression and alow substitution of values using .(var) syntax. for your case do something like:

text( 5,1, bquote( italic(r[M]) == .(rv) ) )