Book Image

R Graph Essentials

Book Image

R Graph Essentials

Overview of this book

Table of Contents (11 chapters)
R Graph Essentials
About the Author
About the Reviewers

Including regression lines

In ggplot, you can include regression lines using geom_abline(). For the next example, we set up the same graph of patient height against weight that we have used several times before:

P <- ggplot(T, aes(x = HEIGHT, y = WEIGHT_1)) + geom_point()

As a start, let's calculate the slope and intercept of the line of best fit (regression line) for height against weight before treatment. In Chapter 1, Base Graphics in R – One Step at a Time, in the section entitled Including a regression line, we saw how to include a linear regression line on a graph. Now, we use the lm() command again to fit a linear regression model by using the following syntax:

lm(WEIGHT_1 ~ HEIGHT, data = T)

Here is the output that you will see on your screen:

lm(formula = WEIGHT_1 ~ HEIGHT, data = T)
(Intercept)       HEIGHT  
   -123.611        1.166  

So, the intercept is approximately -123.61 and the slope is approximately 1.17. You can now include the regression line...