Cannot get minor grid lines to appear in matplotlib figure
Unfortunately, ax.grid is a bit confusing in this regard. (This is a design bug / common gotcha.) It turns the minor grid on, but the minor ticks are still turned off.
What you need to do is call plt.minorticks_on or ax.minorticks_on in addition to calling ax.grid(True, which='both').
You should use plt.minorticks_on().
import matplotlib.pyplot as pltimport numpy as npfig = plt.figure(1)ax = fig.add_subplot(111)x = np.linspace(0,10,41)y = np.sin(x)plt.plot(x,y)plt.grid(b=True, which='major', color='k', linestyle='-')plt.grid(b=True, which='minor', color='r', linestyle='-', alpha=0.2)plt.minorticks_on()plt.show()