To plot a line of best fit in R, use the lm() function to fit a linear model to the data, then plot the model using the plot() function. You can also add the
Approximating the Equation of a Line of Best Fit and Making Predictions, Algebra
Correlation and Dependent t-tests
Constructing a best fit line
How to add best fit lines in scatterplot in R Studio
Curve Fitting using Linear and Nonlinear Regression - Statistics By Jim
Linear Models in R: Plotting Regression Lines - The Analysis Factor
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Chapter 9 Additional advanced plots and customisation options