Extanding the latitude axis of a plot

hey,

I made a graph of H field vs phi (as defined in cylindrical coordinates).
First, i added phi as a claculator's expression, and then draw the graph using rectangular plot.

As seen in the attached picture, Phi is shown for 0 to 180 degrees.
The thing is that i want to extand it to 360 degrees, but i couldn't find how.

thanks a lot !