Plotting of a vector function is e.g. here:
http://fenicsproject.org/documentation/dolfin/1.4.0/python/demo/documented/stokes-iterative/python/documentation.html
In this case, you can use just a very simple plot(u) command.
To see more possibilities, go to the documentation page of plot command.
You will see there more options of this command (at the bottom of the page).
