How to create loop to build matplotlib bar stacked bar graphs using a dataframe?

Question

I am trying to write more efficient code here.

I am trying to build a stacked bar graph of a company's sales for the 4 quarters of 2021.

I am using matplotlib pyplot graph library.

The problem is that to build a stack graph with more than 5 information per stacked bar requires a lot of repeated codes.

I am trying to build a loop to reduce the amount of lines of codes. But I am unable to reach the right result.

Here is what the data looks like:

enter image description here

Here is what I did to do a stack graph:

# Setting up all our subplots
fig, ax = plt.subplots()

# Editing each bars
ax.bar(df21.index, df21["mac_share"], label= "mac_share")
ax.bar(df21.index, df21["service_share"], label = "service_share", bottom = df21["mac_share"])
ax.bar(df21.index, df21["accessories_share"], label = "accessories_share", bottom = df21["service_share"] + df21["mac_share"])
ax.bar(df21.index, df21["ipad_share"], label = "ipad_share", bottom = df21["service_share"] + df21["mac_share"] + df21["accessories_share"])
ax.bar(df21.index, df21["iphone_share"], label = "iphone_share", bottom = df21["service_share"] + df21["mac_share"] + df21["accessories_share"] + df21["ipad_share"])

# Adding graphs element
ax.set_xticklabels(df21.index, rotation=45)
ax.set_ylabel("Sales shares in %")
ax.legend()

# Showing graphs
plt.show()

And here is the result, which is the good one:

enter image description here

However, I think it's too long. So I tried to build a loop around the creation of each bars. But the result isn't what I expected.

Here is the code:

# Setting up all our subplots
fig, ax = plt.subplots()

# Recalling our names of columns
shares = ['mac_share', 'service_share', 'accessories_share', 'ipad_share', 'iphone_share']

# Editing the first bar
ax.bar(df21.index, df21[shares[0]], label= shares[0])

# Looping through our shares list
for i in range(len(shares)-1):
    ax.bar(df21.index, df21[shares[i+1]], label = shares[i+1], bottom =+ df21[shares[i]])

# Adding graphs details
ax.set_xticklabels(df21.index, rotation=45)
ax.set_ylabel("Sales shares in %")
ax.legend()

# plotting my graph
plt.show()

But when I execute the code, here is the result: enter image description here

As you can see, there are many parts of each stackbar missing compared to the original graphs up in the graph.

Do you know how I could write a loop to build this graph?

Thank you in advance!

Andrew · Accepted Answer · 2022-09-12 14:20:06Z

1

Try this for your loop:

# Looping through our shares list
bottom = []
for i in range(len(shares)-1):
    bottom += df21[shares[i]]
    ax.bar(df21.index, df21[shares[i+1]], label = shares[i+1], bottom = bottom)

I think you used =+ when you meant += (note that =+ isn't an operator in python, so it's equivalent to just =), and python doesn't let you assign the result of using += (that is, you can't do something like y = (x += 3)). So just modify bottom separately and then use bottom in you call to ax.bar()

edited Sep 12, 2022 at 14:20

answered Sep 9, 2022 at 14:32

Andrew

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4 Comments

Thefinsloth Over a year ago

Hi @Andrew, thank you so much for your answer. So I tried it but didn't work out how I expected. Maybe I have done some mistakes, but here is what it says:

Input In [52]     ax.bar(df21.index, df21[shares[i+1]], label = shares[i+1], bottom)                                                                ^ SyntaxError: positional argument follows keyword argument

Do you have any clue what I have done wrong?

Andrew Over a year ago

If you google that error, you get this question which links to this related question which explains that in python you have to pass required parameters before optional parameters to a function. Checking the documentation shows the bottom parameter is optional, so do ax.bar(df21.index, df21[shares[i+1]], label = shares[i+1], bottom = bottom). I've updated my answer.

Thefinsloth Over a year ago

Thank you very much! I'll put the answer as the good one once I have enough credit ^^

Andrew Over a year ago

Note that you don't need a certain amount of reputation to mark the answer as accepted, you just can't upvote before you have 15 reputation

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