r / sciplot: overlapping whiskers in lineplot.CI

Question

When making an interaction plot using lineplot.CI in the sciplot library, error bars can overlap across groups. For example,

data = c(1,5,3,7,3,7,5,9)
grp1 = c(1,1,1,1,2,2,2,2)
grp2 = c(1,1,2,2,1,1,2,2)
lineplot.CI(grp1, data, grp2)

The groups can be separated along the x axis by adding jitter to the grouping variable and setting x.cont to TRUE, but this makes the lines in the plot disappear:

data = c(1,5,3,7,3,7,5,9)
grp1 = c(1,1,1,1,2,2,2,2) + c(-0.05, -0.05, 0.05, 0.05, -0.05, -0.05, 0.05, 0.05)
grp2 = c(1,1,2,2,1,1,2,2)
lineplot.CI(grp1, data, grp2, x.cont=TRUE)

Is it possible to get the lines to appear and to jitter the points, so that the error bars don't overlap? Or is there a better way to make this kind of plot?

Answer 1

You can use ggplot2 for that. Here is an example with a built-in dataset (as I don't have your standard errors or CIs). The key is to use position_dodge().

ToothGrowth$dose.cat <- factor(ToothGrowth$dose, labels=paste("d", 1:3, sep=""))
df <- with(ToothGrowth , aggregate(len, list(supp=supp, dose=dose.cat), mean))
df$se <- with(ToothGrowth , aggregate(len, list(supp=supp, dose=dose.cat), 
              function(x) sd(x)/sqrt(10)))[,3]

opar <- theme_update(panel.grid.major = theme_blank(),
                     panel.grid.minor = theme_blank(),
                     panel.background = theme_rect(colour = "black"))

xgap <- position_dodge(0.2)
gp <- ggplot(df, aes(x=dose, y=x, colour=supp, group=supp))
gp + geom_line(aes(linetype=supp), size=.6, position=xgap) + 
     geom_point(aes(shape=supp), size=3, position=xgap) + 
     geom_errorbar(aes(ymax=x+se, ymin=x-se), width=.1, position=xgap)
theme_set(opar)