How to plot a separator line between two data classes?

Question

I have a simple exercise that I am not sure how to do. I have the following data sets:

male100

    Year    Time
0   1896    12.00
1   1900    11.00
2   1904    11.00
3   1906    11.20
4   1908    10.80
5   1912    10.80
6   1920    10.80
7   1924    10.60
8   1928    10.80
9   1932    10.30
10  1936    10.30
11  1948    10.30
12  1952    10.40
13  1956    10.50
14  1960    10.20
15  1964    10.00
16  1968    9.95
17  1972    10.14
18  1976    10.06
19  1980    10.25
20  1984    9.99
21  1988    9.92
22  1992    9.96
23  1996    9.84
24  2000    9.87
25  2004    9.85
26  2008    9.69

and the second one:

female100

    Year    Time
0   1928    12.20
1   1932    11.90
2   1936    11.50
3   1948    11.90
4   1952    11.50
5   1956    11.50
6   1960    11.00
7   1964    11.40
8   1968    11.00
9   1972    11.07
10  1976    11.08
11  1980    11.06
12  1984    10.97
13  1988    10.54
14  1992    10.82
15  1996    10.94
16  2000    11.12
17  2004    10.93
18  2008    10.78

I have the following code:

y = -0.014*male100['Year']+38

plt.plot(male100['Year'],y,'r-',color = 'b')
ax = plt.gca() # gca stands for 'get current axis'
ax = male100.plot(x=0,y=1, kind ='scatter', color='g', label="Mens 100m", ax = ax)
female100.plot(x=0,y=1, kind ='scatter', color='r', label="Womens 100m", ax = ax)

Which produces this result:

I need to plot a line that would go exactly between them. So the line would leave all of the green points below it, and the red point above it. How do I do so?

I've tried playing with the parameters of y, but to no avail. I also tried fitting a linear regression to male100 , female100 , and the merged version of them (across rows), but couldn't get any results.

Any help would be appreciated!

Answer 1

A solution is using support vector machine (SVM). You can find two margins that separate two classes of points. Then, the average line of two support vectors is your answer. Notice that it's happened just when these two set of points are linearly separable.
You can use the following code to see the result:

Data Entry

male = [
(1896  ,  12.00),
(1900  ,  11.00),
(1904  ,  11.00),
(1906  ,  11.20),
(1908  ,  10.80),
(1912  ,  10.80),
(1920  ,  10.80),
(1924  ,  10.60),
(1928  ,  10.80),
(1932  ,  10.30),
(1936  ,  10.30),
(1948  ,  10.30),
(1952  ,  10.40),
(1956  ,  10.50),
(1960  ,  10.20),
(1964  ,  10.00),
(1968  ,  9.95),
(1972  ,  10.14),
(1976  ,  10.06),
(1980  ,  10.25),
(1984  ,  9.99),
(1988  ,  9.92),
(1992  ,  9.96),
(1996  ,  9.84),
(2000  ,  9.87),
(2004  ,  9.85),
(2008  ,  9.69)
        ]
female = [
(1928,    12.20),
(1932,    11.90),
(1936,    11.50),
(1948,    11.90),
(1952,    11.50),
(1956,    11.50),
(1960,    11.00),
(1964,    11.40),
(1968,    11.00),
(1972,    11.07),
(1976,    11.08),
(1980,    11.06),
(1984,    10.97),
(1988,    10.54),
(1992,    10.82),
(1996,    10.94),
(2000,    11.12),
(2004,    10.93),
(2008,    10.78)
]

Main Code

Notice that the value of C is important here. If it is selected to 1, you can't get the preferred result.

from sklearn import svm
import numpy as np
import matplotlib.pyplot as plt

X = np.array(male + female)
Y = np.array([0] * len(male) + [1] * len(female))

# fit the model
clf = svm.SVC(kernel='linear', C=1000) # C is important here
clf.fit(X, Y)
plt.figure(figsize=(8, 4))
# get the separating hyperplane
w = clf.coef_[0]
a = -w[0] / w[1]
xx = np.linspace(-1000, 10000)
yy = a * xx - (clf.intercept_[0]) / w[1]
plt.figure(1, figsize=(4, 3))
plt.clf()
plt.plot(xx, yy, "k-") #********* This is the separator line ************

plt.scatter(X[:, 0], X[:, 1], c=Y, zorder=10, cmap=plt.cm.Paired,
 edgecolors="k")
plt.xlim((1890, 2010))  
plt.ylim((9, 13)) 
plt.show()

Answer 2

I believe your idea of making use of regression lines is correct - if they aren't used, the line would be merely superficial (and impossible to justify if the points overlap in the event of messy data). Therefore, using some randomly made data with a known linear relationship, we can do the following:

import random
import numpy as np
from matplotlib import pyplot as plt
from sklearn.linear_model import LinearRegression

x_values = np.arange(0, 51, 1)

y_points_1 = [i * 2 + random.randint(5, 30) for i in x_points]
y_points_2 = [i - random.randint(5, 30) for i in x_points]

x_points = x_values.reshape(-1, 1)

def regression(x, y):
    model = LinearRegression().fit(x, y)
    y_pred = model.predict(x)
    
    return y_pred

barrier = [(regression(x=x_points, y=y_points_1)[i] + value) / 2 for i, value in enumerate(regression(x=x_points, y=y_points_2))]

plt.plot(x_points, regression(x=x_points, y=y_points_1))
plt.plot(x_points, regression(x=x_points, y=y_points_2))
plt.plot(x_points, barrier)
plt.scatter(x_values, y_points_1)
plt.scatter(x_values, y_points_2)
plt.grid(True)
plt.show()

Giving us the following plot:

This method also works for an overlap in the data points, so if we change the random data slightly and apply the same process:

x_values = np.arange(0, 51, 1)

y_points_1 = [i * 2 + random.randint(-10, 30) for i in x_points]
y_points_2 = [i - random.randint(-10, 30) for i in x_points]

We get something like the following:

It is important to note that the lists used here are of the same length, so you would need to add some predicted points to the female data after applying regression in order to make use of the line between them. These points would merely be along the regression line with the x-values corresponding to those present in the male data.

Answer 3

Because sklearn might be a bit over the top for a linear fit and to get rid of the condition that you would need the same number of data points for male and female data, here the same implementation with numpy.polyfit. This also demonstrates that their approach is not a solution to the problem.

import pandas as pd
import numpy as np
from matplotlib import pyplot as plt

#data import
male = pd.read_csv("test1.txt", delim_whitespace=True)
female = pd.read_csv("test2.txt", delim_whitespace=True)

#linear fit of both populations
pmale   = np.polyfit(male.Year, male.Time, 1)
pfemale = np.polyfit(female.Year, female.Time, 1)

#more appealing presentation, let's pretend we do not just fit a line
x_fitmin=min(male.Year.min(), female.Year.min())
x_fitmax=max(male.Year.max(), female.Year.max())
x_fit=np.linspace(x_fitmin, x_fitmax, 100)

#create functions for the three fit lines
male_fit   = np.poly1d(pmale)
print(male_fit)
female_fit = np.poly1d(pfemale)
print(female_fit)
sep        = np.poly1d(np.mean([pmale, pfemale], axis=0))
print(sep)

#plot all markers and lines
ax = male.plot(x="Year", y="Time", c="blue", kind="scatter", label="male")
female.plot(x="Year", y="Time", c="red", kind="scatter", ax=ax, label="female")
ax.plot(x_fit, male_fit(x_fit), c="blue", ls="dotted", label="male fit")
ax.plot(x_fit, female_fit(x_fit), c="red", ls="dotted", label="female fit")
ax.plot(x_fit, sep(x_fit), c="black", ls="dashed", label="separator")

plt.legend()
plt.show()

Sample output:

-0.01333 x + 36.42
 
-0.01507 x + 40.92
 
-0.0142 x + 38.67

And one point is still in the wrong section. However - I find this question so interesting because I expected answers from the sklearn crowd for non-linear data groups. I even installed sklearn in anticipation! If in the next days nobody posts a good solution with SVMs, I will set a bounty on this question.