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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!

Thefinsloth
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1 Answers1

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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()

Andrew
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  • 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? – Thefinsloth Sep 12 '22 at 09:05
  • If you google that error, you get [this question](https://stackoverflow.com/q/42163846/16662168) which links to this [related question](https://stackoverflow.com/questions/16932825/why-cant-non-default-arguments-follow-default-arguments) which explains that in python you have to pass required parameters before optional parameters to a function. Checking the [documentation](https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.bar.html) 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. – Andrew Sep 12 '22 at 14:26
  • Thank you very much! I'll put the answer as the good one once I have enough credit ^^ – Thefinsloth Sep 13 '22 at 16:57
  • 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 – Andrew Sep 14 '22 at 16:00