Modifying the Grid in matplotlib.pyplot graph

Question

I am a newbie to Python but slowly getting there. I am having a problem trying to increase the number of grid lines on a graph. Basically, the Graph is labelled 0-24 (Hours) but the x axis only generates a label every 5 hours (O,5,10,15,20) with a grid line at each of those majors. Ideally, I would like a grid line every hour as I am collecting real time data.

Most of this code has been lifted from various sources, but the one thing that has stumped me is how to configure the grid..

Edit - As requested my simplified code is below..

import numpy as np
import matplotlib.pyplot as plt
import time

timedata=[0.01,1.1,2.2,3.3,4.4,5.55,6.6,7.7,8.8,9.1,10.2,11.2,12.2,13.2,14.1,15.2,16.1,17.2,18.1,19.2,20.1,21.1,22.2,23.1]
#timedata is in decimal hours
bxdata=[10,10,20,20,20,30,30,30,40,40,40,30,30,30,20,20,30,30,20,20,40,50,30,24]
bydata=[20,10,20,30,20,30,30,30,5,40,40,30,5,30,20,20,30,35,20,20,5,50,30,24]

#draw the graph
fig, ax = plt.subplots(sharex=True, figsize=(12, 6))
x=np.arange(0,24,1)

ax.plot(timedata,bxdata, color='red', label='Bx',lw=1)
ax.plot (timedata, bydata, color='blue', label = 'By',lw=1)
ax.set_xlim(0,24)
ax.set_ylim(-250,250)

plt.ion()
plt.xlabel("Time (Hours)")
plt.ylabel("nT")
plt.grid(True, which='both')
plt.legend()
plt.show()
image = "test.png"
time.sleep(2)
plt.savefig(image)
plt.close('all')

and this is the graph that I get.

Test graph

Hi Martyn, welcome to StackOverflow! Please prepare [minimal, reproducible example](https://stackoverflow.com/help/minimal-reproducible-example). It will be easier for us to help you. Also take a look at [this answer](https://stackoverflow.com/a/48791644/4601890) as it might solve your problem — dankal444, Dec 22 '21 at 21:46
Thank you for the reply. I have looked at the suggested fix. It doesn't do what I need it to do. The graph needs to be preformatted, with X axis and Y axis preformatted over 24 hours. This is my simplified code that works as a piece of test. In reality, I am collecting data samples every 15 seconds but this shows the effect. — Martyn Kinder, Dec 23 '21 at 10:47

score 1 · Answer 1 · answered Dec 23 '21 at 12:49

If you set the x-axis spacing to any desired interval, the grid will automatically be drawn in conjunction with it. There is a mixture of object-oriented and plot formats, so the object format is used for unification.

import numpy as np
import matplotlib.pyplot as plt
import time

timedata=[0.01,1.1,2.2,3.3,4.4,5.55,6.6,7.7,8.8,9.1,10.2,11.2,12.2,13.2,14.1,15.2,16.1,17.2,18.1,19.2,20.1,21.1,22.2,23.1]
#timedata is in decimal hours
bxdata=[10,10,20,20,20,30,30,30,40,40,40,30,30,30,20,20,30,30,20,20,40,50,30,24]
bydata=[20,10,20,30,20,30,30,30,5,40,40,30,5,30,20,20,30,35,20,20,5,50,30,24]

#draw the graph
fig, ax = plt.subplots(sharex=True, figsize=(12, 6))
x=np.arange(0,24,1)

ax.plot(timedata,bxdata, color='red', label='Bx',lw=1)
ax.plot(timedata, bydata, color='blue', label='By',lw=1)
ax.set_xlim(0,24)
ax.set_ylim(-250,250)

# plt.ion()
ax.set_xticks(np.arange(0,24,1))
ax.set_xlabel("Time (Hours)")
ax.set_ylabel("nT")
ax.grid(True, which='both')
ax.legend()

# image = "test.png"
# time.sleep(2)
# plt.savefig(image)
# plt.close('all')

plt.show()

For more information about [coding styles](https://matplotlib.org/stable/tutorials/introductory/usage.html#coding-styles), please refer to this information. If you find my answers helpful, please click the check mark next to the answer to accept the answer. — r-beginners, Dec 23 '21 at 12:56

gboffi · Accepted Answer · 2021-12-23T15:39:06.140

The idea is to associate a locator to the minor x-axis ticks, the locator you need is MultipleLocator and we use it also to fix the major ticks' spacing (for hours, 6 is better than 5, isn't it?)

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.ticker import MultipleLocator

y = np.random.rand(25)
plt.plot(y)

plt.gca().xaxis.set_major_locator(MultipleLocator(6))
plt.gca().xaxis.set_minor_locator(MultipleLocator(1))

plt.grid()
plt.grid(True, 'minor', color='#ddddee') # use a lighter color

plt.show()

Modifying the Grid in matplotlib.pyplot graph

2 Answers2