Why is pandas inserting spaces in my histogram?

Question

Example data can be found here in CSV format.

Given the following code:

figure()
grp.vis.plot(kind='hist', alpha=.5, normed=True)
show()

I obtain the following figure:

Why is pandas inserting gaps in the figure? The values range from 0 to 7, and are all represented, so I see no reason why this should happen.

Thanks very much in advance!

Answer 1

Because parameter bins with default value 10 is in hist:

grp.vis.plot(kind='hist', alpha=.5, bins=7, normed=True)

If omit rwidth:

grp.vis.plot(kind='hist', alpha=.5, bins=7)

Docs:

bins : integer or array_like, optional

If an integer is given, bins + 1 bin edges are returned, consistently with numpy.histogram() for numpy version >= 1.3.

Unequally spaced bins are supported if bins is a sequence.

default is 10

rwidth : scalar or None, optional

The relative width of the bars as a fraction of the bin width. If None, automatically compute the width.

Ignored if histtype is ‘step’ or ‘stepfilled’.

Default is None

Answer 2

Sorry for a bit of off-topic self-promotion, but perhaps you might find useful my library physt (see https://github.com/janpipek/physt ). Among other features, it provides different binning schemas, one of which ("integer") is suited for automatic "bins" for integer data.

import pandas as pd
import physt

df = pd.read_csv("visanal_so.csv")
ax = physt.h1(df.vis, "integer").plot(density=True, alpha=0.5)
ax.set_ylabel("Frequency");

P.S. Note that the plot is similar to the original but dissimilar to what @jezrael shows. The automatic pandas binning behaves somewhat strange and definitely not in the way you intended.