如何在matplotlib中的网格线之间打勾？

Question

In my simple example below, how to make x-axis tick values to appear between grids?

import numpy as np
import matplotlib.pyplot as plt

np.random.seed(1)
x = range(10)
y = np.random.random(10)
plt.plot(x,y)
plt.xticks(x)
plt.grid(True)
plt.show()

The following make ticks to be where I want but the grid lines also moves.

np.random.seed(1)
x = range(10)
y = np.random.random(10)
plt.plot(x,y)
plt.xticks(x)
plt.grid(True)
plt.xticks(np.arange(10)+0.5, x)
plt.show()

I would like the result to be:

Answer 1

You can set the minor ticks so that only 1 minor tick appears inbetween your major ticks. This is done using matplotlib.ticker.AutoMinorLocator . Then, set the gridlines to only appear at the minor ticks. You also need to shift your xtick positions by 0.5:

from matplotlib.ticker import AutoMinorLocator

np.random.seed(10)

x = range(10)
y = np.random.random(10)
plt.plot(x,y)
plt.xticks(np.arange(0.5,10.5,1), x)
plt.xlim(0,9.5)
plt.ylim(0,1)
minor_locator = AutoMinorLocator(2)
plt.gca().xaxis.set_minor_locator(minor_locator)
plt.grid(which='minor')

plt.show()

Edit : I'm having trouble getting two AutoMinorLocator s to work on the same axis. When trying to add in another one for the y axis, the minor ticks get messed up. A work around I have found is to manually set the locations of the minor ticks using a matplotlib.ticker.FixedLocator and passing in the locations of the minor ticks.

from matplotlib.ticker import AutoMinorLocator
from matplotlib.ticker import FixedLocator
np.random.seed(10)

x = range(10)
y = np.random.random(10)
plt.plot(x,y)
plt.xticks(np.arange(0.5,10.5,1), x)
plt.yticks([0.05,0.15,0.25,0.35,0.45,0.55,0.65,0.75,0.85,0.95,1.05], [0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,1])
plt.xlim(0,9.5)
plt.ylim(0,1.05)

minor_locator1 = AutoMinorLocator(2)
minor_locator2 = FixedLocator([0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9,1])
plt.gca().xaxis.set_minor_locator(minor_locator1)
plt.gca().yaxis.set_minor_locator(minor_locator2)
plt.grid(which='minor')

plt.show()

Answer 2

If you use plt.subplots for figure creation, you get an axes object, too:

f, ax = plt.subplots(1)

This one has a better Interface for adjusting grid/ticks. Then you can give explicitly x-values for your data shifted 0.5 to the left. The same do with the minor ticks and let the grid be shown at the minor ticks:

f, ax = plt.subplots(1)
ax.set_xticks(range(10))
x_values = np.arange(10) - .5
ax.plot(x_values, np.random.random(10))
ax.set_xticks(x_values, minor=True)
ax.grid(which='minor')