CODEWITHSUNDEEP
pythondataframematplotlib
user11914512
user11914512

Reputation:

How to get the minimum (or maximum) boundary of a scatter plot in Matplotlib

Say I have thousands of random (x, y) data points and I store xs and ys in two columns in a dataframe. It is important to note that all xs are integers, but ys are continuous numbers. If I plot them in a scatter plot using Matplotlib, it looks like below. Now I want to get the minimum boundary of the plot, which I depicted in a red curve. How should I do it? To make it clear, I want to get the indices for (x,y) pairs with the minimum y value for each x, so the length of the indices should be equal to len(set(x)).

enter image description here

Upvotes: 2

Views: 2968

Answers (2)

tomjn
tomjn

Reputation: 5389

Is df.grouby('x').min() what you want?

A full example:

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

df = pd.DataFrame({'x': np.random.randint(10, size=1000), 'y': np.random.rand(1000)})
df.plot.scatter('x', 'y', color='k')
df.groupby('x').min().plot(ax=plt.gca(), color='red')

To get the indices of the original dataframe you can use idxmin on the groupby e.g.

df.groupby('x').idxmin()

Upvotes: 1

Jonas Pirner
Jonas Pirner

Reputation: 152

try: 

import matplotlib.pyplot as plt
import pandas as pd

df = pd.DataFrame({'A':[1,2,3,1,2,3],'B':[1.3,2.6,3.2,5.6,4.5,3.1]})
fig, ax = plt.subplots()
ax.plot(df.A, df.B, '-')
temp = df.groupby('A')['B'].min().reset_index()
ax.plot(temp.A, temp.B, 'r-')

Upvotes: 0

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