How to draw pandas dataframe using Matplotlib hist with multiple y axes

Question

I have a dataframe as below:

frame_id time_stamp    pixels  step
0        50   06:34:10  0.000000       0
1       100   06:38:20  0.000000       0
2       150   06:42:30  3.770903       1
3       200   06:46:40  3.312285       1
4       250   06:50:50  3.077356       0
5       300   06:55:00  2.862603       0

I want to draw two y-axes in one plot. One is for pixels. The other is for step. x-axis is time_stamp. I want the plot for step like the green line like this:

Answer 1

Here's an example that could help. Change d1 and d2 as per your variables and the respective labels as well.

import numpy as np                   
import matplotlib.pyplot as plt      

rng = np.random.default_rng(seed=0)

d1 = rng.normal(loc=20, scale=5, size=200)
d2 = rng.normal(loc=30, scale=5, size=500)

fig, ax1 = plt.subplots(figsize=(9,5))

#create twin axes
ax2 = ax1.twinx()

ax1.hist([d1], bins=15, histtype='barstacked', linewidth=2,
        alpha=0.7)
ax2.hist([d2], bins=15, histtype='step', linewidth=2,
        alpha=0.7)

ax1.set_xlabel('Interval')
ax1.set_ylabel('d1 freq')
ax2.set_ylabel('d2 freq')

plt.show()

Getting the bar labels is not easy with the two types of histograms in the same plot using matplotlib.

bar_labels

Answer 2

Instead of histograms you could use bar plots to get the desired output. I have also added in a function to help get the bar labels.

import matplotlib.pyplot as plt
import numpy as np
time = ['06:34:10','06:38:20','06:42:30','06:46:40','06:50:50','06:55:00']

step = [0,0,1,1,0,0]
pixl = [0.00,0.00,3.77,3.31,3.077,2.862]

#function to add labels
def addlabels(x,y):
    for i in range(len(x)):
        plt.text(i, y[i], y[i], ha = 'center')
fig, ax1 = plt.subplots(figsize=(9,5))

#generate twin axes
ax2 = ax1.twinx()

ax1.step(time,step, 'k',where="mid",linewidth=1)

ax2.bar(time,pixl,linewidth=1)

addlabels(time,step)
addlabels(time,pixl)
ax1.set_xlabel('Time')
ax1.set_ylabel('Step')
ax2.set_ylabel('Pixels')

plt.show()

bar_plot