How to add a title to represent four different plot in lm function

Dear All,

A linear regression model could be fitted by using lm function and the plot function can be used to check the
assumptions of the model. The example is as followed.

require(graphics)
## Annette Dobson (1990) "An Introduction to Generalized Linear Models".
## Page 9: Plant Weight Data.
ctl <- c(4.17,5.58,5.18,6.11,4.50,4.61,5.17,4.53,5.33,5.14)
trt <- c(4.81,4.17,4.41,3.59,5.87,3.83,6.03,4.89,4.32,4.69)
group <- gl(2,10,20, labels=c("Ctl","Trt"))
weight <- c(ctl, trt)
anova(lm.D9 <- lm(weight ~ group))
opar <- par(mfrow = c(2,2), oma = c(0, 0, 1.1, 0))
plot(lm.D9, las = 1)     

Could someone advice me the way to add a single title either at the above or bottom of these 4 plots, entitled
"The verification of model assumtion via four different plots" ?

Thanks
Fir