plot a function with a changing parameter

Hi everybody, I would like to plot the function y=a*x, with a=parameter. What I want is to plot y and x, and then, I don't know, an (interactive) animation of this graph when "a" changes, or add the possibility of change the values of "a" in real time. I hope I have explained well what I'm looking for. Thanks for your attention.

Answer 1

[Nikola]

Ahh man, I need this so bad right now...