To view a phase-plane portrait of an ODE vector field, simply
run the program pplane in the Matlab command window.

>> pplane
>>

This should open an applet that allows you to input a two
dimensional autonomous vector field. This has the form
x' = f(x,y)
y' = g(x,y)

Here, x and y are scalars, and the two functions, f(x,y) and
g(x,y) comprise the vector field whose integral curves are the
solutions of the ODE. The applet will plot the vector field on
the Cartesian plane y vs. x. You have the option to choose the
x-min and x-max values and the same for the y-axis.

If you left-click with the mouse on the plot, then pplane
will integrate the ODE forward and backward in time with
the point you clicked as the initial condition. If the vector
field has singular points, then the integrator can "get stuck"
and you should click on the "stop" button on the right-hand-
side of the plot.

The applet also allows you to choose parameters, which is
convenient for situations where one wants to integrate a
vector field for several values of some parameters. For
instance, in the example
x' = a*x + b*sin(y)  + c
y' =  d*x