If I gave you the equation x = 1 to plot, how would you plot it, after all, I didn't give you any y values!

You'd plot a vertical straight line that crosses the x axis at 1 and continues through negative infinity to infinity in the y direction!

So you have to do the same here. When you plot an equation, you are plotting ALL possibly values that satisfy that equation. In this case, there are an infinite number of values of r that satisfy that equation. So basically, your polar graph is a straight line the has an infinite r, and makes an angle if with the x axis!