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Understanding Scatter Plots with Numpy: Ensuring Same Size X and Y Arrays

Rajiv Chandra
Name
Rajiv Chandra

Updated on

A fundamental aspect of data visualization in Python is creating scatter plots, especially when dealing with machine learning algorithms like K-Means. However, it can be quite a common occurrence to encounter issues with different-sized numpy arrays when plotting these graphs. This article provides a detailed explanation on how to align the sizes of your X and Y numpy arrays for efficient scatter plotting.

Background: The Issue

Imagine a situation where you have 37 numpy arrays, divided into two separate arrays, X and Y, with 18 and 19 elements, respectively. You try to plot these values using matplotlib's scatter function. However, you are faced with a ValueError stating, "x and y must be the same size". This is because the scatter function requires both X and Y arrays to have the same number of elements.

The Core Problem

The scatter function, a powerful tool in matplotlib, plots two variables against each other. It requires an equal number of data points in both the X and Y arrays. The error arises when these arrays don't align in size, as in our case, where X has 18 and Y has 19 columns.

A Failed Attempt to Resolve

One might consider zipping two columns from X into one to make the sizes equal. However, this leads to another issue: the new column in X is a numpy array, not a float like the other columns. Consequently, this results in another ValueError: "setting an array element with a sequence."

The Resolution: Plotting All Elements

The solution lies in understanding that scatter plots are essentially two-dimensional. They represent relationships between two variables, not 37. Thus, it's unfeasible to plot all 37 arrays directly on a scatter plot.

However, to visualize your data before applying the K-Means algorithm, you can use pairplot from the seaborn library, which allows you to plot pairwise relationships in a dataset. This way, you can inspect the relationships and distributions of each pair of your 37 arrays.

import seaborn as sns
import pandas as pd
 
# Assuming f1, f2, ..., f37 are your 1D numpy arrays
df = pd.DataFrame(list(zip(f1, f2, f3, ..., f37)), columns=['f1', 'f2', 'f3', ..., 'f37'])
sns.pairplot(df)

This code will generate a grid of Axes such that each variable in your data will be shared across the y-axes across a single row and the x-axes across a single column.

The Lesson

The crux of the matter is that scatter plots are limited to two dimensions. While it's tempting to view all data points simultaneously, doing so can lead to confusion and erroneous results. Instead, opt for pair plots, or consider reducing dimensionality with techniques like PCA if you're working with high-dimensional data.

Remember, the key to successful data visualization is not just in the complexity of the plot, but in the clarity and insights it provides.