Matplotlib plot multiple bars in one graph

Question

I have a plot with multiple bars with different scenarios, but when I plot it all the bars are repeated. Please find below my code.

I know that I'm using only one value at a time from the list, but when I try to pass the whole sub-array using data[0] instead, I get a Value mismatch error:

ValueError: shape mismatch: objects cannot be broadcast to a single shape

What am I doing wrong? I looked at the PyPlot example and this other post and both pass an array to ax.bar.

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

data = [[20, 35, 30, 40], [25, 40, 45, 30], 
        [15, 20, 35, 45], [10, 25, 40, 15], 
        [50, 20, 45, 55], [10, 55, 60, 20]]
data_std = [[1, 2, 1, 2], [1, 2, 1, 2], [1, 2, 1, 2], 
            [1, 2, 1, 2], [1, 2, 1, 2], [1, 2, 1, 2]]    

length = len(data)
x_labels = ['A', 'B', 'C', 'D', 'E', 'F']

# Set plot parameters
fig, ax = plt.subplots()
width = 0.2 # width of bar
x = np.arange(length)

ax.bar(x, data[0][0], width, color='#000080', label='Case-1', yerr=data_std[0][0])
ax.bar(x + width, data[0][1], width, color='#0F52BA', label='Case-2', yerr=data_std[0][1])
ax.bar(x + (2 * width), data[0][2], width, color='#6593F5', label='Case-3', yerr=data_std[0][2])
ax.bar(x + (3 * width), data[0][3], width, color='#73C2FB', label='Case-4', yerr=data_std[0][3])

ax.set_ylabel('Metric')
ax.set_ylim(0,75)
ax.set_xticks(x + width + width/2)
ax.set_xticklabels(x_labels)
ax.set_xlabel('Scenario')
ax.set_title('Title')
ax.legend()
plt.grid(True, 'major', 'y', ls='--', lw=.5, c='k', alpha=.3)

fig.tight_layout()
plt.show()

Result is:

What would be the expected outcome when you "pass the whole sub-array using data[0] instead"? This does not seem to make much sense, so maybe you want to explain what you would like to see your plot to look like? — ImportanceOfBeingErnest
– ImportanceOfBeingErnest, Commented Nov 16, 2018 at 16:56
@ImportanceOfBeingErnest I updated the post with the current plot. As you can see, all the bars are the same for every scenario. I'd like to change the code to plot the other values as well — user9518022
– user9518022, Commented Nov 16, 2018 at 17:00

ImportanceOfBeingErnest · Accepted Answer · 2018-11-16 17:07:01Z

You want to plot the data column-wise. Hence it makes sense to convert the lists to arrays and select the respective column to plot.

import numpy as np
import matplotlib.pyplot as plt

data = np.array([[20, 35, 30, 40], [25, 40, 45, 30], 
                 [15, 20, 35, 45], [10, 25, 40, 15], 
                 [50, 20, 45, 55], [10, 55, 60, 20]])
data_std = np.array([[1, 2, 1, 2], [1, 2, 1, 2], [1, 2, 1, 2], 
                     [1, 2, 1, 2], [1, 2, 1, 2], [1, 2, 1, 2]])    

length = len(data)
x_labels = ['A', 'B', 'C', 'D', 'E', 'F']

# Set plot parameters
fig, ax = plt.subplots()
width = 0.2 # width of bar
x = np.arange(length)

ax.bar(x, data[:,0], width, color='#000080', label='Case-1', yerr=data_std[:,0])
ax.bar(x + width, data[:,1], width, color='#0F52BA', label='Case-2', yerr=data_std[:,1])
ax.bar(x + (2 * width), data[:,2], width, color='#6593F5', label='Case-3', yerr=data_std[:,2])
ax.bar(x + (3 * width), data[:,3], width, color='#73C2FB', label='Case-4', yerr=data_std[:,3])

ax.set_ylabel('Metric')
ax.set_ylim(0,75)
ax.set_xticks(x + width + width/2)
ax.set_xticklabels(x_labels)
ax.set_xlabel('Scenario')
ax.set_title('Title')
ax.legend()
plt.grid(True, 'major', 'y', ls='--', lw=.5, c='k', alpha=.3)

fig.tight_layout()
plt.show()

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