geom_hline(mapping = NULL, data = NULL, stat = "hline", position = "identity", show_guide = FALSE, ...)
aes_string. Only needs to be set at the layer level if you are overriding the plot defaults.
layer. This can include aesthetics whose values you want to set, not map. See
layerfor more details.
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):
p + geom_hline(yintercept=20)
# 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)