Contour plot with python

Question

This is my code. I tried using it to plot the contour, however, I get two plots on the same grid. exactly half instead of a full plot. I mean my grid is 20 units in x direction however, my resulting plot is 10 units in x direction with replica plot from 10 to 20 units. Can you please look whats wrong? v is a numpy array of 20x10

x, y = range(0, 20), range(0,10)
xi, yi = np.meshgrid(x, y)


# Interpolate
rbf = scipy.interpolate.Rbf(xi, yi, v)
zi = rbf(xi, yi)

plt.imshow(zi, vmin = 1.6, vmax=2)
plt.colorbar()
plt.show()