Horizontal line.


geom_hline(mapping = NULL, data = NULL, stat = "hline", position = "identity", show_guide = FALSE,


should a legend be drawn? (defaults to FALSE)
The aesthetic mapping, usually constructed with aes or aes_string. Only needs to be set at the layer level if you are overriding the plot defaults.
A layer specific dataset - only needed if you want to override the plot defaults.
The statistical transformation to use on the data for this layer.
The position adjustment to use for overlappling points on this layer
other arguments passed on to layer. This can include aesthetics whose values you want to set, not map. See layer for more details.


This geom allows you to annotate the plot with horizontal lines (see geom_vline and geom_abline for other types of lines).


There are two ways to use it. You can either specify the intercept of the line in the call to the geom, in which case the line will be in the same position in every panel. Alternatively, you can supply a different intercept for each panel using a data.frame. See the examples for the differences


geom_hline understands the following aesthetics (required aesthetics are in bold):

  • alpha
  • colour
  • linetype
  • size


p <- ggplot(mtcars, aes(x = wt, y=mpg)) + geom_point() p + geom_hline(aes(yintercept=mpg))

p + geom_hline(yintercept=20)

p + geom_hline(yintercept=seq(10, 30, by=5))

# With coordinate transforms p + geom_hline(aes(yintercept=mpg)) + coord_equal()

p + geom_hline(aes(yintercept=mpg)) + coord_flip()

p + geom_hline(aes(yintercept=mpg)) + coord_polar()

# To display different lines in different facets, you need to # create a data frame. p <- qplot(mpg, wt, data=mtcars, facets = vs ~ am) hline.data <- data.frame(z = 1:4, vs = c(0,0,1,1), am = c(0,1,0,1)) p + geom_hline(aes(yintercept = z), hline.data)

See also

geom_vline for vertical lines, geom_abline for lines defined by a slope and intercept, geom_segment for a more general approach