subplot titles in R

I'm new in R. I have created 9x3 subplots using par(mfrow= c(3,3)) command using a simple loop and plotted scatter plot for pair-wise dependent variable. Now I want to have sub titles through (a-c) for 1st row; (d-f) for 2nd row & (g-i) for the last. Using "sub" 开发者_如何转开发command I could get (a-c) for 1st row but unable to get (d-f) for 2nd row & successively. Can anybody let me know in what way I can do so?

Each plot call can create its own title using the main= assignment. The tricky step is getting a "master title". For that you will probably want to use mtext. Well OK, I suppose there may be some tricks to getting the subtitles you want with R "expressions", but that would be better illustrated if you offer a dataset and code than discussing in the abstract.