## 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:

Call:lm(formula = WEIGHT_1 ~ HEIGHT, data = T)Coefficients:(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...