There are several methods to perform this task I am sharing a few with you:
Use par, it will plot on the same graph but axis will be different. E.x.
plot( x, y1, type="l", col="blue" )
plot( x, y2, type="l", col="yellow" )
You can add lines() or points() to the existing graph but it won’t create a new window.
Use matplot function, e.x.
There are some other methods too, I hope one of this help you with your issue.