4.8 Add Auxiliary Lines

Having learned how to generate scatterplots, smoothline fits, and line plot, it is sometimes helpful to add auxiliary lines to existing plots to provide additional information.

4.8.1 Using `abline()` with `plot()`

Let’s first review the scatterplot between liv_area and sale_price.

library(r02pro)
plot(sahp$liv_area, sahp$sale_price)

In this plot, you may want to add some auxiliary lines. You can use the function abline() after a call of plot() to do this. To add a vertical line, you can set the parameter v; to add a horizontal line, you can set the parameter h.

plot(sahp$liv_area, sahp$sale_price)
abline(v = 2000, col = "purple")       ##Add a vertical line at liv_area = 2000

plot(sahp$liv_area, sahp$sale_price)
abline(h = 300, col = "blue")         ##Add a horizontal line at sale_price = 2000

Note that the v and h arguments can also be vectors with more than one values, which will lead to multiple vertical or horizontal lines. The corresponding argument col can also be a vector with multiple values.

plot(sahp$liv_area, sahp$sale_price)
abline(h = c(100, 200, 300), 
       col = c("red","blue","green"))         ##Add multiple horizontal lines

In addition to adding vertical lines and horizontal lines, you can also add any line with the abline() function. We know a line can be represented as a function \(y = a + b\times x\), where \(a\) is the intercept and \(b\) is the slope. In the abline(), you can generate such a line by specifying the parameter a for the intercept and b for the slope. Note that you can run abline() multiple times to add multiple lines.

plot(sahp$liv_area, sahp$sale_price)
abline(h = 300, col = "blue")   
abline(a = 100, b = 0.1, col = "green")

4.8.2 Using `geom_hline()`, `geom_vline()` and `geom_abline()`

Let’s first review the following scatterplot between liv_area and sale_price.

library(r02pro)
library(tidyverse)
ggplot(data = sahp) + 
  geom_point(mapping = aes(x = liv_area, y = sale_price))

Looking at the scatterplot, it maybe helpful to add a horizontal line. To do this, you can use the geom_hline() function with argument yintercept specifying the value on the y-axis.

ggplot(data = sahp) + 
  geom_point(mapping = aes(x = liv_area, y = sale_price)) + 
  geom_hline(yintercept = 300, color = "red")

Here, a horizontal line at 300 is added to the scatterplot.

You can also add both vertical lines and horizontal lines to the same plot.

ggplot(data = sahp) + 
  geom_point(mapping = aes(x = liv_area, y = sale_price)) + 
  geom_vline(xintercept = 2000, color = "green") + 
  geom_hline(yintercept = 300, color = "red")

In addition to adding vertical lines and horizontal lines, you can also add any line with the geom_abline() function. We know a line can be represented as a function \(y = a + b\times x\), where \(a\) is the intercept and \(b\) is the slope. In the geom_abline(), you can generate such a line by specifying the slope and intercept arguments.

ggplot(data = sahp) + 
  geom_point(mapping = aes(x = liv_area, y = sale_price)) + 
  geom_abline(slope = 0.1, intercept = 100, color = "blue")

Similar to smoothline fit and line plots, you can change line type here. Different numbers correspond to different line types.

ggplot(data = sahp) + 
  geom_point(mapping = aes(x = liv_area, y = sale_price)) + 
  geom_abline(slope = 0.1, intercept = 100, linetype = 3)

4.8.3 Exercises

Using the sahp data, using plot() to generate a scatterplot between lot_area (x-axis) and sale_price (y-axis), and add horizontal lines at 150 with color “blue” and 200 with color “red.”
Using the sahp data, using ggplot() to generate a scatterplot between sale_price (x-axis) and liv_area (y-axis), and add the following lines to the plot:

\(y = 5\times x + 1000\) (in green color, dashed line)
\(y = 3\times x + 1500\) (in purple color, solid line)