How to plot correlation between two columns

Question

The task is the following:

Is there a correlation between the age of an athlete and his result at the Olympics in the entire dataset?

Each athlete has a name, age, medal (gold, silver, bronze or NA).

In my opinion, it is necessary to count the number of all athletes of the same age and calculate the percentage of them who have any kind of medal (data.Medal.notnull()). The graph should show all ages on the x-axis, and the percentage of those who has any medal on the y-axis. How to get this data and create the graphic with help of pandas and matprolib?

For instance, some data like in table:

Name Age Medal
Name1 20 Silver 
Name2 21 NA
Name3 20 NA
Name4 22 Bronze
Name5 22 NA
Name6 21 NA
Name7 20 Gold
Name8 19 Silver
Name9 20 Gold
Name10 20 NA
Name11 21 Silver

The result should be (in the graphic):

19 - 100%
20 - 60%
21 - 33%
22 - 50%

ouroboros1 · Accepted Answer · 2022-10-01 16:21:40Z

3

First, turn df.Medal into 1s for a medal and 0s for NaN values using np.where.

import pandas as pd
import numpy as np

data = {'Name': {0: 'Name1', 1: 'Name2', 2: 'Name3', 3: 'Name4', 4: 'Name5', 
                 5: 'Name6', 6: 'Name7', 7: 'Name8', 8: 'Name9', 9: 'Name10', 
                 10: 'Name11'}, 
        'Age': {0: 20, 1: 21, 2: 20, 3: 22, 4: 22, 5: 21, 6: 20, 7: 19, 8: 20, 
                9: 20, 10: 21}, 
        'Medal': {0: 'Silver', 1: np.nan, 2: np.nan, 3: 'Bronze', 4: np.nan, 
                  5: np.nan, 6: 'Gold', 7: 'Silver', 8: 'Gold', 9: np.nan, 
                  10: 'Silver'}}

df = pd.DataFrame(data)

df.Medal = np.where(df.Medal.notna(),1,0)

print(df)

      Name  Age  Medal
0    Name1   20      1
1    Name2   21      0
2    Name3   20      0
3    Name4   22      1
4    Name5   22      0
5    Name6   21      0
6    Name7   20      1
7    Name8   19      1
8    Name9   20      1
9   Name10   20      0
10  Name11   21      1

Now, you could plot the data maybe as follows:

import seaborn as sns
import matplotlib.ticker as mtick

sns.set_theme()
ax = sns.barplot(data=df, x=df.Age, y=df.Medal, errorbar=None)

# in versions prior to `seaborn 0.12` use
# `ax = sns.barplot(data=df, x=df.Age, y=df.Medal, ci=None)`

ax.yaxis.set_major_formatter(mtick.PercentFormatter(xmax=1.0))

# adding labels
ax.bar_label(ax.containers[0], 
             labels=[f'{round(v*100,2)}%' for v in ax.containers[0].datavalues])

Result:

Incidentally, if you would have wanted to calculate these percentages, one option could have been to use pd.crosstab:

percentages = pd.crosstab(df.Age,df.Medal, normalize='index')\
    .rename(columns={1:'percentages'})['percentages']
print(percentages)

Age
19    1.000000
20    0.600000
21    0.333333
22    0.500000
Name: percentages, dtype: float64

So, with matplotlib, you could also do something like:

percentages = pd.crosstab(df.Age,df.Medal, normalize='index')\
    .rename(columns={1:'percentages'})['percentages'].mul(100)
    
my_cmap = plt.get_cmap("viridis")
rescale = lambda y: (y - np.min(y)) / (np.max(y) - np.min(y))

fig, ax = plt.subplots()

ax.bar(x=percentages.index.astype(str), 
             height=percentages.to_numpy(), 
             color=my_cmap(rescale(percentages.to_numpy())))

ax.yaxis.set_major_formatter(mtick.PercentFormatter())
ax.bar_label(ax.containers[0], fmt='%.1f%%')
    
plt.show()

Result:

edited Oct 1, 2022 at 16:21

answered Oct 1, 2022 at 13:23

ouroboros1

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12 Comments

Toshik Over a year ago

Thank you. This solution works, but there was the error here: "ax = sns.barplot(data=df, x=df.Age, y=df.Medal, errorbar=None)". The error: "Pass the following variable{} as {}keyword arg{}: {}. From version 0.12, the only valid positional argument". I removed "errorbar=None" and now it works, but shows black lines. How to remove black lines? Screeshot: imgur.com/a/KOpyVq1

Toshik Over a year ago

As I understood, it's an errorbar. But it isn't necessary, it's just required to show ordinary bars.

Toshik Over a year ago

Wow, I have just updated Seaborn in Anaconda Prompt and now it works. Yes, your code is absolutely correct, and errorbar=None works, and blacklines (erorbars) are removed. It's strange because I downloaded Seaborn yestarday. The version should have been the latest. My request for moderators is to highlight my solution of this problem in case of other people will have problem with 'errorbar'. ouroboros1, I really appreciate your effort!

ouroboros1 Over a year ago

@TrentonMcKinney: Yes, I realized that, except I didn't get the percentage to work. E.g. if I do: ax.bar_label(ax.containers[0], fmt='%.1f%%'), you'll get 1.0% instead of 100.0%. Any idea how one would fix that? Although, come to think of it, this is not a problem indeed for the matplotlib solution, cos I could of course, simply add .mul(100) to the series percentages. I'll update that at least.

ouroboros1 Over a year ago

@TrentonMcKinney: figured out one way to do this for the seaborn solution as well: [f'{round(v*100,2)}%' for v in ax.containers[0].datavalues] for the labels, though would be nice if there was a simple way just with fmt. Thanks for the comment.

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