Here is another option for plotting. The streamline_plot does a nice job of plotting some sample curves along with arrows to give direction. I like to use the odeint solver since it works for systems so nicely. Here I use it by defining a system $ds/dt = 1, dx/dt = x^2-4x$. This gives the appropriate plot. Note that we have to tell Sage that the number 1 should be in the symbolic ring (SR) to get this to work.
var('x s t')