Optimsing x-axis according to data range - plt.yticks` between the horizontal bars

Question

I am trying to generate population plot using code below. I re used some of code I found. However, I dont know I can optimise legends according to the data range I have. I mean so I should have nice plot which is squished because of wrong x.axis limit.

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

age = np.array(["0-9", "10-19", "20-29", "30-39", "40-49", "50-59", "60-69", "70-79", "80-89", '90-99',
                "100-109", "110-119", "120-129", "130-139", "140-150", ">150"])
m = np.array([811, 34598, 356160, 381160, 243330, 206113, 128549, 60722, 8757, 1029, 1033, 891, 1803, 62, 92, 764])
f = np.array(
    [612, 101187, 904717, 841066, 503661, 421678, 248888, 95928, 10289, 1444, 1360, 1377, 1699, 119, 173, 1655])
x = np.arange(age.size)
tick_lab = ['3M', '2M', '1M', '1M', '2M', '3M']
tick_val = [-3000000, -2000000, -1000000, 1000000, 2000000, 3000000]


plt.figure(figsize=(16, 8), dpi=80)
def plot_pyramid():
    plt.barh(x, -m, alpha=.75, height=.75, left=-shift, align='center' , color="deepskyblue")
    plt.barh(x, f, alpha=.75, height=.75, left = shift, align='center', color="pink")
    plt.yticks([])
    plt.xticks(tick_val, tick_lab)
    plt.grid(b=False)
    plt.title("Population Pyramid")
    for i, j in enumerate(age):
        if i == 0 or i==1:
            plt.text(-150000, x[i] - 0.2, j, fontsize=14)
        else:    
            plt.text(-230000, x[i] - 0.2, j, fontsize=14)


if __name__ == '__main__':
    plot_pyramid()

Any help will be appreciated

Thanks in advance

JohanC · Accepted Answer · 2021-01-22T22:39:16.983

Here are a few ideas to tackle the issues mentioned:

Instead of putting the xticks at fixed positions, let matplotlib choose automatically where to put the ticks.
A custom tick formatter could display the numbers either with M or with K depending on their size.
The labels for the age ranges could be placed centered instead of left aligned.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.ticker import FuncFormatter

age = np.array(["0-9", "10-19", "20-29", "30-39", "40-49", "50-59", "60-69", "70-79", "80-89", '90-99',
                "100-109", "110-119", "120-129", "130-139", "140-150", ">150"])
m = np.array([811, 34598, 356160, 381160, 243330, 206113, 128549, 60722, 8757, 1029, 1033, 891, 1803, 62, 92, 764])
f = np.array([612, 101187, 904717, 841066, 503661, 421678, 248888, 95928, 10289, 1444, 1360, 1377, 1699, 119, 173, 1655])
x = np.arange(age.size)

def k_and_m_formatter(x, pos):
    if x == 0:
        return ''
    x = abs(x)
    if x > 900000:
        return f'{x / 1000000: .0f} M'
    elif x > 9000:
        return f'{x / 1000: .0f} K'
    else:
        return f'{x : .0f}'

def plot_pyramid():
    fig, ax = plt.subplots(figsize=(16, 8), dpi=80)
    shift = 0
    ax.barh(x, -m, alpha=.75, height=.75, left=-shift, align='center' , color="deepskyblue")
    ax.barh(x, f, alpha=.75, height=.75, left = shift, align='center', color="pink")
    ax.set_yticks([])
    ax.xaxis.set_major_formatter(FuncFormatter(k_and_m_formatter))
    ax.grid(b=False)
    ax.set_title("Population Pyramid")
    for i, age_span in enumerate(age):
        ax.text(0, x[i], age_span, fontsize=14, ha='center', va='center')

plot_pyramid()

Optionally the x-axis could be log scaled (ax.xscale('symlog')) to make the small values more visible.

The code in the question uses a variable shift, but doesn't give it a value. Giving it a value different from 0 will result in a gap (to place the labels?), but would also place the ticks on the wrong positions.

For an alternative, see e.g. How to build a population pyramid? or Using Python libraries to plot two horizontal bar charts sharing same y axis for an example with two separate subplots.

Optimsing x-axis according to data range - plt.yticks` between the horizontal bars

1 Answers1