Python matplotlib restrict to integer tick locations

Question

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?

Asked By: Epimetheus

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Source

Answer 1

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()

Answered By: Epimetheus

Answer 2

You can use the MaxNLocator method, like so:

    from pylab import MaxNLocator

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

Answered By: Florian Mayer

Answer 3

 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.

Answered By: Aliabbas Petiwala

Answer 4

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))

Answered By: Andy Hsu

Python matplotlib restrict to integer tick locations

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