Adding abline of different data to xyplot panels

Question

Using lattice in R, I have plotted an xyplot using the factors option. Now I want to add a horizontal abline to every plot, however out of another dataframe. How can I get these

Answer 1

I think you have to use a custom panel function.

library(lattice)

MyData <- data.frame(
  v1 = 1:50,
  v2 = rnorm(50,20,5),
  v3 = c(rep("one", 10), rep("two", 10),  rep("three", 10), rep("four", 10), rep("five", 10)))

lineData <- c(30,30,20,20,40) 
# define custom panel plot function
customPanel = function(x, y, lineData) {
  panel.xyplot(x, y)
  panel.abline(h = lineData[panel.number()]) # instead of using a global variable
}

xyplot(
  v1 ~ v2 | v3,
  data = MyData,
  layout = c(5, 1),
  scales = list(
    x = list(relation="free"),
    y = list(relation="same")
  ),
  panel = customPanel,
  lineData = lineData)

Answer 2

Here's how i'd do it to make sure the values match up

lineData <- data.frame(
  v3 = c("one","two","three","four","five"),
  height = c(30,30,20,20,40)
)

xyplot(
  v1 ~ v2 | v3,
  data = MyData,
  layout = c(5, 1),
  scales = list(
    x = list(relation="free"),
    y = list(relation="same")
  ),
  panel = function(x, ...) {
      level <- dimnames(trellis.last.object())[["v3"]][packet.number()]
      panel.xyplot(x, ...);
      panel.abline(h = lineData$height[lineData$v3==level])
  }
)

This produces