This page plots a system of differential equations of the form dx/dt = f(x,y), dy/dt = g(x,y).
For a much more sophisticated phase plane plotter, see the MATLAB plotter written by John C. Polking of Rice University.

The direction field solver knows about trigonometric, logarithmic and exponential functions, but multiplication and evaluation must be entered explicitly (2*x and sin(x), not 2x and sin x). 
Minimum x:  Minimum y:  Arrow length:  Variable length arrows  
Maximum x:  Maximum y:  Number of arrows:  