Matplotlib: enforce equal size (height) of subplots?

I have a figure containing three subplots. The first subplot is an image (imshow), while the other two are distributions (plot).

Here's the code:

# collects data
imgdata = ...      # img of shape  (800, 1600, 3)    
x = ...            # one 1600-dimensional vector
y = ...            # one  800-dimensional vector

# create the figure 
f = plt.figure()    

# create subplots   
subplot_dim = (1, 3)
p_img = plt.subplot2grid(subplot_dim, (0, 0), aspect="auto")
p_x = plt.subplot2grid(subplot_dim, (0, 1), aspect="auto")
p_y = plt.subplot2grid(subplot_dim, (0, 2), aspect="auto")

p_img.imshow(imgdata, interpolation="None")
p_x.plot(x)         
p_y.plot(y)         


# save figure       
f.set_size_inches(21.0, 12.0)
f.savefig("some/path/image.pdf", dpi=80)

My problem is, that the two subplots p_x, p_y always have a greater height than the image subplot p_img.

Thus, the result always looks like this:

                  ###############   ###############   
                  #             #   #  *****      #
                  #        *****#   # *     *     #
###############   #     ***     #   # *      *    #
#             #   #    *        #   # *        *  #
#    image    #   #   *         #   #*           *#
#             #   #***          #   #*           *#
###############   ###############   ###############
     p_img              p_x               p_y

How can I enforce an equal size (or at least height) of p_img, p_x and p_y?