R: How to join multiple plots into one plot?

I have two density distribution data A and B, I want to plot the two density curves into one plot. I used the following codes:

plot(user.dens.tas, col="red", xlim=c(0,0.003))
plot(item.dens.tas, col="blue", xlim=c(0,0.003))

How can I plot the two curves into one plot (combine them)?

Answer

plot(user.dens.tas, col="red", xlim=c(0,0.003))
lines(item.dens.tas, col="blue", xlim=c(0,0.003))

Puts one line in read, the other in blue on the same plot. You might also consider adding a title and other presentation niceties.