how to draw an arrow, draw a circle?

Hi everybody,
Can you help me about two problems?

1. how to draw an arrow



i'm looking for drawing single arrow (not an arrow field with quiver).
i saw there is an arrow class in the module matplotlib.patches.
So, i create an instance of the class like this:
a= Arrow(0,0,5,5,2.0)
The problem is i don't know how to draw the arrow on a figure.

My goas is to draw several arrows. Each origin of arrows is placed on the x axis. (i.e y=0)
The head of the arrow vary on x and y axes.
Here is an example:
/-----------------> x
         \ /

2. how to draw a circle?

i'm trying to generate the smith chart which is very usefull in electromagnetic domain.
In this purpose, i need to draw circle.
I try to find in the polar function the way circles are drawn but i didn't find it.
Can you told me John how did you draw circles in the polar figure?

Philippe Collet