I want to write something like (LaTeX style) b_{norm}=\frac{F\prime(0)}{R\prime(0)} how do I add the "prime" (first derivative) to a R-plot? The help of "plotmath" just talks about "partialdiff". Can you complete this command? text( 30,0.05,labels=expression(b[plain("norm")]==frac(F(0),R(0))) ) Thanks, Thomas