[R] How to draw a border for multiple graphs in one page

Hi, I am struggling to create a 2 by 2 multiple graphs in one page. I used par(mfrow=c(2,2)) to divide the screen into 4. In each screen I draw a pie chart (They are all same). For example, my data is like this Concentration value A1 69 A2

[R] How to draw a border for multiple graphs in one page

Hi, I am struggling to create a 2 by 2 multiple graphs in one page. I used par(mfrow=c(2,2)) to divide the screen into 4. In each screen I draw a pie chart (They are all same). For example, my data is like this Concentration value A1 69 A2