Adding a line on top of each bar in MatPlotlib graph

Question

I have the code for following graph. I need to add a threshold value for each of the graph. This value does change from one group to another.

import numpy as np
import matplotlib.pyplot as plt

n_groups = 4
heuristic = (230.193, 33.96, 46, 8)
safe = (195.8, 24.83, 36, 7)

# create plot
fig, ax = plt.subplots()
index = np.arange(n_groups)
bar_width = 0.1
opacity = 0.8

rects1 = plt.bar(index, heuristic, bar_width,
             marker="D",
             alpha=opacity,
             color='b',
             label='Heuristic')

rects2 = plt.bar(index + bar_width, safe, bar_width,
             alpha=opacity,
             color='g',
             label='SAFE')

plt.xlabel('Firmware')
plt.ylabel('nDCG')
plt.title('Matching results by firmware')
plt.xticks(index + bar_width, ('Mqtt', 'Solder', 'Iron', 'Sympetrum'))
plt.legend()

plt.tight_layout()
plt.show()

The following is the graph it produces and I have used pink marker to show you what I want the code to produce. Sorry the lines are not straight. Any help is really appreciated. Thanks

Scott Boston · Accepted Answer · 2020-11-21 20:11:14Z

Here's a way using hlines:

import numpy as np
import matplotlib.pyplot as plt

n_groups = 4
heuristic = (230.193, 33.96, 46, 8)
safe = (195.8, 24.83, 36, 7)
threshold = (250, 50, 80, 30)

# create plot
fig, ax = plt.subplots()
index = np.arange(n_groups)
bar_width = 0.1
opacity = 0.8

rects1 = plt.bar(index, heuristic, bar_width,
             #marker="D",
             alpha=opacity,
             color='b',
             label='Heuristic')

rects2 = plt.bar(index + bar_width, safe, bar_width,
             alpha=opacity,
             color='g',
             label='SAFE')

[
    ax.hlines(
        threshold[i],
        index[i] - bar_width / 2,
        index[i] + bar_width * 1.5,
        colors="deeppink",
    )
    for i in range(len(safe))
]

plt.xlabel('Firmware')
plt.ylabel('nDCG')
plt.title('Matching results by firmware')
plt.xticks(index + bar_width, ('Mqtt', 'Solder', 'Iron', 'Sympetrum'))
plt.legend()

plt.tight_layout()
plt.show()

Output:

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Adding a line on top of each bar in MatPlotlib graph

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