How can I label a span of values on matplotlib axis?

Question

In my histogram, I need to annotate the X-axis like that:

The "0" label is spanning between two ticks since there is a separate bin for 0s only in the histogram, while rest of the axis is linear to indicate boundaries of other bins. The underbrace is not crucial, but there shall be some indicator that "0" spans for entire bin.

The closest solution I found so far is "draw it yourself" (How to add group labels for bar charts in matplotlib?). I am looking rather for .axvspan() equivalent for axis (as .axvline() is for ticks), like something intended for labelling .axvspan().

Answer 1

There is no ".axvspan() equivalent for axis". Also ".axvline()" is not "for ticks". And there is also nothing intended for labeling .axvspan().

So let's answer the question how to produce some red bracket (paranthesis) below the plot. You can basically just plot a line in form of the bracket and position it according to the xaxis transform. Below is a function to do that.

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
ax.bar(range(-1,5),range(3,9), width=1, align="edge", ec="k", alpha=1)
ax.set_xticks(range(1,6))


def bracket(ax, pos=[0,0], scalex=1, scaley=1, text="",textkw = {}, linekw = {}):
    x = np.array([0, 0.05, 0.45,0.5])
    y = np.array([0,-0.01,-0.01,-0.02])
    x = np.concatenate((x,x+0.5)) 
    y = np.concatenate((y,y[::-1]))
    ax.plot(x*scalex+pos[0], y*scaley+pos[1], clip_on=False, 
            transform=ax.get_xaxis_transform(), **linekw)
    ax.text(pos[0]+0.5*scalex, (y.min()-0.01)*scaley+pos[1], text, 
                transform=ax.get_xaxis_transform(),
                ha="center", va="top", **textkw)

bracket(ax, text="0", pos=[-1,-0.01], linekw=dict(color="crimson", lw=2) )
plt.show()