CODEWITHSUNDEEP
pythonmatplotlibplotlogarithm
NeutronStar
NeutronStar

Reputation: 2157

Python, matplotlib: how to set tick label values to their logarithmic values

I have some data that I plot on a semi-log plot (log-lin style, with a logarithmic scale on the y-axis). Is there a way to change the y-axis tick labels from their actual values to their logarithmic values?

As an example, consider the following code:

import matplotlib.pyplot as plt
import numpy as np

x=np.array([1,2,3,4,5])
def f(x):
    return 10**(x-1)

plt.plot(x,f(x))
plt.yscale(u'log')

plt.show()

Which produces the following plot: enter image description here

(Sorry it is kind of big, I do not know how to make it smaller, feel free to edit to help out with that).

In this plot the tick labels are shown as 10^0, 10^1, 10^2, etc.; however I would like them to display as their logarithmic values: 0, 1, 2, etc.

I realize I could go back and change plt.plot(x,f(x)) to plt.plot(x,np.log10(f(x))) and then make the y-axis linear again instead of logarithmic but I want to know if there is anyway matplotlib can just change the y-axis tick values themselves without me having to put np.log10() in all my plt.plot()'s. My reason for this is two-fold: I have many plt.plot() lines in my code and would rather not go back and have to change it for all of them, and then I wouldn't have logarithmically spaced minor ticks (although I'm sure there's some way to change that even with a linear axis).

EDIT: I am aware of this question which has some similarities to mine but is not the same. The person in that question wants to change the tick labels from scientific form to "normal" decimal form. I want to change my tick labels from scientific form to the logarithmic (base 10) value of the number. I am sure the answer will be similar to the one I linked but it is not obvious to me how to do it. In fact, I looked at that question before posting mine but still decided to post mine because I did not know how to apply it to my problem. Perhaps to experienced programmers it is obvious how to apply the methods of the question I linked to my situation but it isn't obvious to me so please step me through it.

If you could show me a code sample (by copying my code sample and putting in the necessary lines) how this works I would much appreciate it.

Upvotes: 3

Views: 4550

Answers (1)

Nelz11
Nelz11

Reputation: 3266

You can use a custom formatter, for example:

import matplotlib.pyplot as plt
from matplotlib.ticker import FuncFormatter
import numpy as np
import math

x=np.array([1,2,3,4,5])

def f(x):
    return 10**(x-1)

plt.plot(x,f(x))
plt.yscale(u'log')

#SET CUSTORM TICK FORMATTING
plt.gca().yaxis.set_major_formatter(FuncFormatter(lambda x,y: '{}'.format(math.log(x, 10))))

plt.show()

Upvotes: 4

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