Python matplotlib restrict to integer tick locations

Quite often I want to make a bar chart of counts. If the counts are low I often get major and/or minor tick locations that are not integers. How can I prevent this? It makes no sense to have a tick at 1.5 when the data are counts.

This is my first attempt:

import pylab
pylab.figure()
ax = pylab.subplot(2, 2, 1)
pylab.bar(range(1,4), range(1,4), align='center')
major_tick_locs = ax.yaxis.get_majorticklocs()
if len(major_tick_locs) < 2 or major_tick_locs[1] - major_tick_locs[0] < 1:
    ax.yaxis.set_major_locator(pylab.MultipleLocator(1))
minor_tick_locs = ax.yaxis.get_minorticklocs()
if len(minor_tick_locs) < 2 or minor_tick_locs[1] - minor_tick_locs[0] < 1:
    ax.yaxis.set_minor_locator(pylab.MultipleLocator(1))

which works OK when the counts are small but when they are large, I get many many minor ticks:

import pylab
ax = pylab.subplot(2, 2, 2)
pylab.bar(range(1,4), range(100,400,100), align='center')
major_tick_locs = ax.yaxis.get_majorticklocs()
if len(major_tick_locs) < 2 or major_tick_locs[1] - major_tick_locs[0] < 1:
    ax.yaxis.set_major_locator(pylab.MultipleLocator(1))
minor_tick_locs = ax.yaxis.get_minorticklocs()
if len(minor_tick_locs) < 2 or minor_tick_locs[1] - minor_tick_locs[0] < 1:
    ax.yaxis.set_minor_locator(pylab.MultipleLocator(1))

How can I get the desired behaviour from the first example with small counts whilst avoiding what happens in the second?

Answers:

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Method 1

You can use the MaxNLocator method, like so:

    from pylab import MaxNLocator

    ya = axes.get_yaxis()
    ya.set_major_locator(MaxNLocator(integer=True))

Method 2

I had a similar issue with a histogram I was plotting showing fractional count. Here’s how I was able to resolve it:

plt.hist(x=[Dataset being counted])

# Get your current y-ticks (loc is an array of your current y-tick elements)
loc, labels = plt.yticks()

# This sets your y-ticks to the specified range at whole number intervals
plt.yticks(np.arange(0, max(loc), step=1))

Method 3

I think it turns out I can just ignore the minor ticks. I’m going to give this a go and see if it stands up in all use cases:

def ticks_restrict_to_integer(axis):
    """Restrict the ticks on the given axis to be at least integer,
    that is no half ticks at 1.5 for example.
    """
    from matplotlib.ticker import MultipleLocator
    major_tick_locs = axis.get_majorticklocs()
    if len(major_tick_locs) < 2 or major_tick_locs[1] - major_tick_locs[0] < 1:
        axis.set_major_locator(MultipleLocator(1))

def _test_restrict_to_integer():
    pylab.figure()
    ax = pylab.subplot(1, 2, 1)
    pylab.bar(range(1,4), range(1,4), align='center')
    ticks_restrict_to_integer(ax.xaxis)
    ticks_restrict_to_integer(ax.yaxis)

    ax = pylab.subplot(1, 2, 2)
    pylab.bar(range(1,4), range(100,400,100), align='center')
    ticks_restrict_to_integer(ax.xaxis)
    ticks_restrict_to_integer(ax.yaxis)

_test_restrict_to_integer()
pylab.show()

Method 4

 pylab.bar(range(1,4), range(1,4), align='center')

and

 xticks(range(1,40),range(1,40))

has worked in my code.
Just use the align optional parameter and xticks does the magic.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0