ggplot2 geom_rug() produces different line length with wide plot

This followed hadley's current previous geom_rug code, but modified it to add (or subtract) an absolute amount for interior units of the rug-ticks. It's really an application of the grid::unit-function more than anything else, since it uses the fact that units can be added and subtracted with different bases. You could modify it to accept a "rug_len"-argument with a default of your choosing, say unit(0.5, "cm"). (Do need to remember to set the environment of the function, so that one closure, geom_rug2, can call the next closure, ggplot2::'+', correctly.)

geom_rug2 <- function (mapping = NULL, data = NULL, stat = "identity", position = "identity", sides = "bl", ...) {
  GeomRug2$new(mapping = mapping, data = data, stat = stat, position = position, sides = sides, ...)
}

GeomRug2 <- proto(ggplot2:::Geom, {
  objname <- "rug2"

  draw <- function(., data, scales, coordinates, sides, ...) {
    rugs <- list()
    data <- coord_transform(coordinates, data, scales)
    if (!is.null(data$x)) {
      if(grepl("b", sides)) {
        rugs$x_b <- segmentsGrob(
          x0 = unit(data$x, "native"), x1 = unit(data$x, "native"),
          y0 = unit(0, "npc"), y1 = unit(0, "npc")+unit(1, "cm"),
          gp = gpar(col = alpha(data$colour, data$alpha), lty = data$linetype, lwd = data$size * .pt)
        )
      }

      if(grepl("t", sides)) {
        rugs$x_t <- segmentsGrob(
          x0 = unit(data$x, "native"), x1 = unit(data$x, "native"),
          y0 = unit(1, "npc"), y1 = unit(1, "npc")-unit(1, "cm"),
          gp = gpar(col = alpha(data$colour, data$alpha), lty = data$linetype, lwd = data$size * .pt)
        )
      }
    }

    if (!is.null(data$y)) {
      if(grepl("l", sides)) {
        rugs$y_l <- segmentsGrob(
          y0 = unit(data$y, "native"), y1 = unit(data$y, "native"),
          x0 = unit(0, "npc"), x1 = unit(0, "npc")+unit(1, "cm"),
          gp = gpar(col = alpha(data$colour, data$alpha), lty = data$linetype, lwd = data$size * .pt)
        )
      }

      if(grepl("r", sides)) {
        rugs$y_r <- segmentsGrob(
          y0 = unit(data$y, "native"), y1 = unit(data$y, "native"),
          x0 = unit(1, "npc"), x1 = unit(1, "npc")-unit(1, "cm"),
          gp = gpar(col = alpha(data$colour, data$alpha), lty = data$linetype, lwd = data$size * .pt)
        )
      }
    }

    gTree(children = do.call("gList", rugs))
  }

  default_stat <- function(.) StatIdentity
  default_aes <- function(.) aes(colour="black", size=0.5, linetype=1, alpha = NA)
  guide_geom <- function(.) "path"
})
environment(geom_rug2) <- environment(ggplot)

p <- qplot(x,y)
p + geom_rug2(size=.1)

With your code creating a png I get:

I'm not sure if there's a way to control the rug segment length in geom_rug (I couldn't find one). However, you can create your own rug using geom_segment and hard-code the segment lengths or add some logic to programatically produce equal-length rug lines. For example:

# Aspect ratio
ar = 0.33

# Distance from lowest value to start of rug segment
dist = 2

# Rug length factor
rlf = 2.5

ggplot(swiss, aes(Education, Fertility)) + geom_point() + 
  geom_segment(aes(y=Fertility, yend=Fertility, 
                   x=min(swiss$Education) - rlf*ar*dist, xend=min(swiss$Education) - ar*dist)) +
  geom_segment(aes(y=min(swiss$Fertility) - rlf*dist, yend=min(swiss$Fertility) - dist, 
                   x=Education, xend=Education)) +
  coord_fixed(ratio=ar,
              xlim=c(min(swiss$Education) - rlf*ar*dist, 1.03*max(swiss$Education)),
              ylim=c(min(swiss$Fertility) - rlf*dist, 1.03*max(swiss$Fertility)))     

Or if you just want to hard-code it:

ggplot(swiss, aes(Education, Fertility)) + geom_point() + 
  geom_segment(aes(y=Fertility, yend=Fertility, 
                   x=min(swiss$Education) - 3, xend=min(swiss$Education) - 1.5)) +
  geom_segment(aes(y=min(swiss$Fertility) - 6, yend=min(swiss$Fertility) - 3, 
                   x=Education, xend=Education)) +
  coord_cartesian(xlim=c(min(swiss$Education) - 3, 1.03*max(swiss$Education)),
                  ylim=c(min(swiss$Fertility) - 6, 1.03*max(swiss$Fertility))) 

As of ggplot2 v3.2.0 you can pass a length argument to geom_rug() to specify the absolute length of the rug:

library(ggplot2)
library(ggthemes)
png(width=800, height=400)
ggplot(swiss, aes(Education, Fertility)) + geom_point() + geom_rug(length = unit(0.5,"cm"))
dev.off()