Matplotlib transparent line plots

I am plotting two similar trajectories in matplotlib and I'd like to plot each of the lines with partial transparency so that the red (plotted second) doesn't obscure the blue.

EDIT: Here's the image with transparent lines.

Plain and simple:

plt.plot(x, y, 'r-', alpha=0.7)

(I know I add nothing new, but the straightforward answer should be visible).

After I plotted all the lines, I was able to set the transparency of all of them as follows:

for l in fig_field.gca().lines:
    l.set_alpha(.7)

EDIT: please see Joe's answer in the comments.

It really depends on what functions you're using to plot the lines, but try see if the on you're using takes an alpha value and set it to something like 0.5. If that doesn't work, try get the line objects and set their alpha values directly.