使用matplotlib为水平条形图创建替代的y轴标签

Question

This is a (clearer) repost of a question I just made, my deidentification was confusing people.

Hi, I'm new to graphing in python using matplotlib.pyplot and spent a good amount of time searching on this site and others, and trying to figure this out but I haven't succeeded in accomplishing what I'd like to do.

Using the code below, I have generated the following graph, which is a collection of courses and the percent of those enrolled who went on to complete the course: What my code is creating:

However, what I would like to generate with python is something like the following (crudely made in MS paint to convey to you all what I would like my code to generate):

What I want to create:

The right hand side axis labels is the number of people who enrolled in each course, which is the additional information I would like featured. An alternative solution is to have the enrollment numbers featured within the graph, written on/at the end of the bars. But whatever works is fine.

import matplotlib.pyplot as plt
y = ['Physics','Chemistry','Economics','Philosophy','Computer Science','Sociology','Biology','History','Overall']
x = [0.0,33.333333333333336,50.0,50.0,50.0,54.54545454545455,58.333333333333336,100.0,51.851851851851855]
alternativeYlabels = ['54', '1', '3', '12', '12', '2', '11', '12', '1']
plt.barh(y,x)
plt.title('Percent Completion Rate of Each Course')

plt.show()

How can I alter my code to fit my needs? Also, how could I add vertical grid lines in my plot corresponding to each 20% in here as well?

Thanks

Answer 1

Here are both the solutions: You need to add a twinx axis. Add the following lines before your plt.show() . If you want to put the values on top the bars instead of the second y-axis on right hand side, you can have a look at this similar answer which I provided few weeks ago (I attached the code below as well along with the output).

ax.xaxis.grid(True) # adding vertical grid lines
ax2 = ax.twinx() # Creating the right hand side y axis
ax_lim = ax.get_ylim()
ax2_lim = (ax_lim[0], ax_lim[1]) # Aligning the limits of both y axes
ax2.set_ylim(ax2_lim)
ax2.set_yticks(range(0, len(y)))
ax2.set_yticklabels(alternativeYlabels[::-1]) # Reverses the list
ax2.set_ylabel("# enrolled in each course",rotation = 270, labelpad=15)

Output

Alternative desired output (see the link in the answer) EDIT: vertical alignment of text based on @ImportanceOfBeingErnest's suggestion

ax.xaxis.grid(True)
for i in range(len(x)):
    ax.text(x[i]+1, i, alternativeYlabels[::-1][i], color='black', fontweight='bold', va="center")

Output

Answer 2

I adapted the code from this matplotlib example which has two separate scales on both sides of the plot. The main difference is that here the second bar plot is a dummy one (each bar has a height of 0). I think this is the easiest way to do it (without manually adding ticks etc.)

# Initialize parameters as in your post
import matplotlib.pyplot as plt
y = ['Physics','Chemistry','Economics','Philosophy','Computer Science','Sociology','Biology','History','Overall']
x = [0.0,33.333333333333336,50.0,50.0,50.0,54.54545454545455,58.333333333333336,100.0,51.851851851851855]
alternativeYlabels = ['54', '1', '3', '12', '12', '2', '11', '12', '1']

# This time draw the bar plot from an 'axes' object
ax1 = plt.subplot(111)
ax1.barh(y,x)

# Draw a second 'dummy' barplot just to define the second ticks
ax2 = ax1.twinx()
ax2.barh(range(len(x)), [0]*len(x), tick_label=alternativeYlabels[::-1])  # don't forget to reverse the labels 

# Add the vertical grid
ax1.xaxis.grid(True)

# Add a title and show figure
plt.title('Percent Completion Rate of Each Course')
plt.show()

Running the code below should produce this figure: