How to fix bar width in this matplotlib plot

Question

Could someone please tell me how to modify this Python code that uses matplotlib such that the width of the bars will stay constant regardless of how many scores are plotted? Thank you in advance!

# data to plot
  n_groups = len(math_scores)
  ###print "im here", math_scores,verbal_scores
  scores_readingwriting = verbal_scores
  scores_math = math_scores

# create plot
  fig, ax = plot.subplots()
  index = np.arange(n_groups)
  bar_width = 0.35
  opacity = 0.8

  rects1 = plot.bar(index, scores_readingwriting, bar_width,
                 alpha=opacity,
                 color='black',
                 label='R/W')

  rects2 = plot.bar(index + bar_width, scores_math, bar_width,
                 alpha=opacity,
                 color='grey',
                 label='Math')

  plot.xlabel('Date',size='14')
  plot.ylabel('Scores',size='14')

  plot.title(str(first_names[i])+' '+str(last_names[i])+"'s History",size='17')
  num=len(scores)
  ###print datesofinterest
  plot.xticks(index + bar_width/2, datesofinterest,size='12')
  plot.yticks(size='12')
  axes = plot.gca()
  axes.set_ylim([200,800])
  plot.legend()

  plot.tight_layout()
  fig.savefig('img'+str(student_ids[i])+'.png')

ImportanceOfBeingErnest · Accepted Answer · 2017-06-03 08:15:27Z

3

The bars are the same width! The reason they don't look the same is because you are using this line:

index = np.arange(n_groups)

This will change your x-values and hence the scale of your x-axis. To counteract this effect you can either change the limits of your x-axis, or make the bar width depend on the number of scores, so for example if a width of 0.35 works for 10 scores then if you double the number of scores you would half the bar width (and vice versa). You can see how either changing the axis limits or bar width gives bars that look the same width:

import numpy as np
import matplotlib.pyplot as plt

# make some dummy data
scores_reading = np.random.randint(50,100,10)
scores_math = np.random.randint(50,100,10)

fig = plt.figure(figsize=(16,5))
bar_width = 0.35
index = np.arange(10)

ax1 = fig.add_subplot(1,4,1)
ax1.bar(index, scores_reading, bar_width, fc='b', edgecolor='none')
ax1.bar(index+bar_width, scores_math, bar_width, fc='r', edgecolor='none')
ax1.set_xlim(0,10)
ax1.set_title('Original - 10 scores')

ax2 = fig.add_subplot(1,4,2)
ax2.bar(index[:5], scores_reading[:5], bar_width, fc='b', edgecolor='none')
ax2.bar(index[:5]+bar_width, scores_math[:5], bar_width, fc='r', edgecolor='none')
ax2.set_title('Original - 5 scores')

ax3 = fig.add_subplot(1,4,3)
ax3.bar(index[:5], scores_reading[:5], bar_width, fc='b', edgecolor='none')
ax3.bar(index[:5]+bar_width, scores_math[:5], bar_width, fc='r', edgecolor='none')
ax3.set_xlim(0,10)
ax3.set_title('Changed limit - 5 scores')

ax4 = fig.add_subplot(1,4,4)
ax4.bar(index[:5], scores_reading[:5], bar_width/2., fc='b', edgecolor='none')
ax4.bar(index[:5]+bar_width, scores_math[:5], bar_width/2., fc='r', edgecolor='none')
ax4.set_title('Changed width - 5 scores')

fig.show()

edited Jun 3, 2017 at 8:15

ImportanceOfBeingErnest

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answered Jun 3, 2017 at 6:23

Robbie

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3 Comments

Laura Holewa Over a year ago

Thank you! I checked that this worked. My only follow-up question would be how do you make the bars in the 2nd bar plot with 5 scores closer together? The bars should always remain touching. Thank you again for your help. :)

Femi Oladeji Over a year ago

The mistake you made was with the red bar. You added bar_width to index[:5] but the size of blue bar is bar_width/2. so this line should be like this ax4.bar(index[:5]+(bar_width/2), scores_math[:5], bar_width/2., fc='r', edgecolor='none')

Robbie Over a year ago

Thanks @FemiOladeji - that would move the bars so they remain touching :-)

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