pyplot zooming in

This question has nothing to do with how you manipulate pyfits, but simply a matter of adding

plt.xlim(1.3, 4.0)

to your code before plt.show()

Use the plt.axis() function with your limits.

plt.axis([x_min, x_max, y_min, y_max])

where x_min, x_max, y_min, and y_max are the coordinate limits for both axes.