http://boisdebout53.org/?syleyman1=site-de-rencontre-philippines-gratuit&03a=00 2nd axis in R
Use axis. side is where the second axis goes. Side 3 is the top x-axis. Side 4 is the right y-axis. With put the 1st axis thick for the at and lab is for the text.
Here is the example.
axis(side=3, at = c(0.1, 0.2, 0.3, 0.4, 0.5), lab = c(10, 5, 3, 2.5, 2))