How to prevent line to extend across whole graph

Question

Currently, the below code (part of a more comprehensive code) generates a line that ranges from the very left to the very right of the graph.

geom_abline(intercept=-8.3, slope=1/1.415, col = "black", size = 1,
          lty="longdash", lwd=1) +

However, I would like the line to only range from x=1 to x=9; the limits of the x-axis are 1-9.

In ggplot2, is there a command to reduce a line that is derived from a manually defined intercept and slope to only cover the range of the x-axis value limits?

Answer 1

You could use geom_segment instead of geom_abline if you want to manually define the line. If your slope is derived from the dataset you are plotting from, the easiest thing to do is use stat_smooth with method = "lm".

Here is an example with some toy data:

set.seed(16)
x = runif(100, 1, 9)
y = -8.3 + (1/1.415)*x + rnorm(100)

dat = data.frame(x, y)

Estimate intercept and slope:

coef(lm(y~x))

(Intercept)           x 
 -8.3218990   0.7036189 

First make the plot with geom_abline for comparison:

ggplot(dat, aes(x, y)) +
    geom_point() +
    geom_abline(intercept = -8.32, slope = 0.704) +
    xlim(1, 9)

Using geom_segment instead, have to define the start and end of the line for both x and y. Make sure line is truncated between 1 and 9 on the x axis.

ggplot(dat, aes(x, y)) +
    geom_point() +
    geom_segment(aes(x = 1, xend = 9, y = -8.32 + .704, yend = -8.32 + .704*9)) +
    xlim(1, 9)

Using stat_smooth. This will draw the line only within the range of the explanatory variable by default.

ggplot(dat, aes(x, y)) +
    geom_point() +
    stat_smooth(method = "lm", se = FALSE, color = "black") +
    xlim(1, 9)