regression - How do I explain the line of best fit in this diagram? - Cross Validated

I teach an intro statistics class at my university (as a graduate student) and I was scouring the internet for interesting graphs on the history of linear regression when I came upon this picture

Chapter 8 Regression II: linear regression

Tutorial: K Fold Cross Validation

Tutorial: Learning Curves for Machine Learning in Python for Data Science

Does linear regression mean the model fit line has to be straight? - Cross Validated

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Machine Learning Google for Developers

Random Forest Regression - The Definitive Guide

1.2 - What is the Best Fitting Line?

How Cross-Validation Works In Machine Learning - Dataaspirant

4. Regression and Prediction - Practical Statistics for Data Scientists [Book]