Same answer as what Walter says, with the additional examples of when you call "hold off" or "cla".
"hold" is off by default. If you put hold on then "the old plots remain and the new plots get attached to it" is what you'll get. If you don't want that, then call "hold off" or "cla".
An even more drastic way to initialize the axes control is:
Nothing (no old plots or axes settings) should remain after you call that.