Matplotlib log scale tick label number formatting

Solution 1:

Sure, just change the formatter.

For example, if we have this plot:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.axis([1, 10000, 1, 100000])
ax.loglog()

plt.show()

You could set the tick labels manually, but then the tick locations and labels would be fixed when you zoom/pan/etc. Therefore, it's best to change the formatter. By default, a logarithmic scale uses a LogFormatter, which will format the values in scientific notation. To change the formatter to the default for linear axes (ScalarFormatter) use e.g.

from matplotlib.ticker import ScalarFormatter
for axis in [ax.xaxis, ax.yaxis]:
    axis.set_major_formatter(ScalarFormatter())

Solution 2:

I've found that using ScalarFormatter is great if all your tick values are greater than or equal to 1. However, if you have a tick at a number <1, the ScalarFormatter prints the tick label as 0.

We can use a FuncFormatter from the matplotlib ticker module to fix this issue. The simplest way to do this is with a lambda function and the g format specifier (thanks to @lenz in comments).

import matplotlib.ticker as ticker

ax.yaxis.set_major_formatter(ticker.FuncFormatter(lambda y, _: '{:g}'.format(y)))

Note in my original answer I didn't use the g format, instead I came up with this lambda function with FuncFormatter to set numbers >= 1 to their integer value, and numbers <1 to their decimal value, with the minimum number of decimal places required (i.e. 0.1, 0.01, 0.001, etc). It assumes that you are only setting ticks on the base10 values.

import matplotlib.ticker as ticker
import numpy as np

ax.yaxis.set_major_formatter(ticker.FuncFormatter(lambda y,pos: ('{{:.{:1d}f}}'.format(int(np.maximum(-np.log10(y),0)))).format(y)))

For clarity, here's that lambda function written out in a more verbose, but also more understandable, way:

def myLogFormat(y,pos):
    # Find the number of decimal places required
    decimalplaces = int(np.maximum(-np.log10(y),0))     # =0 for numbers >=1
    # Insert that number into a format string
    formatstring = '{{:.{:1d}f}}'.format(decimalplaces)
    # Return the formatted tick label
    return formatstring.format(y)

ax.yaxis.set_major_formatter(ticker.FuncFormatter(myLogFormat))