Linear Regression Explained Visually | Slope, Residuals, Gradient Descent & R²

Linear regression visualised from scratch in 4 minutes — scatter plots built point by point, residuals drawn live, gradient descent rolling down the MSE curve in real time, and a degree-9 polynomial that confidently reports R² = 1.00 on training data before completely falling apart on a single new point. If you've ever used LinearRegression().fit() without fully understanding what's happening under the hood — what the slope actually means, why MSE is shap