Width of error bars in line plot using ggplot2 - r

Question

I have some data with standard errors associated and would like to display these with error bars. That's what I have:
# generate some data
hod <- data.frame(h = c(1:24,1:24,1:24), mean = 1:(24*3) + runif(24*3, 0, 5),ci = runif(24*3, 0, 2), t = c(rep("a",24),rep("b",24),rep("c",24)))
pd <- position_dodge(0.3)
dayplot <- ggplot(hod, aes(x=h, y=mean, colour=as.factor(t),group=as.factor(t))) +
geom_line(position=pd, size=1) +
geom_errorbar(aes(ymin=mean-ci, ymax=mean+ci),
width=1,
size=0.5,
position=pd) +
geom_point(position=pd, shape=21, size=1, fill="white") +
scale_x_continuous(limits=c(-0.5,23.5),
breaks=c(0:8*3),
labels=ifelse(
c(0:8*3) < 10,
paste('0',c(0:8*3),':00',sep=''),
paste(c(0:8*3),':00',sep='')
)
) +
xlab("Hour of day") + ylab(ylabel) + labs(title = varlabels[var]) +
theme_minimal() +
theme(plot.margin = unit(c(1,0,1,1), "cm"),
axis.title.x = element_text(vjust=-1),
axis.title.y = element_text(angle=90, vjust=0),
legend.margin = unit(c(0), "cm"),
legend.key.height = unit(c(0.9), "cm"),
panel.grid.major = element_line(colour=rgb(0.87,0.87,0.87)),
panel.grid.minor = element_blank(),
plot.background = element_rect(fill = rgb(0.97,0.97,0.97), linetype=0)
)
The only thing of interest is probably:
geom_errorbar(aes(ymin=mean-ci, ymax=mean+ci),
width=1,
size=0.5,
position=pd)
It gives:
Now when I group the data by a factor variable (as.factor(t)), I get several lines instead of one, which is what I want, BUT, as you can see, the horizontal lines at the error bars are more narrow, and I can't figure out why. I tried changing and even taking away the width and size attribute of geom_errorbar, but nothing happens. Is there a way to have the same width of the horizontal lines for every chart, no matter the data? I mean, why should it vary? Or does that width convey some information?

Answer 1

Below is a reproducible example using random data. The fix to the problem is to multiply the width by the number of classes/factors that you have. In the plot below, since I used three factors, using a width of 3 fixes the problem. ggplot2 seems to calculate the relative width by the number of data points in your dataset, rather than the numeric values on the x-axis. This is (IMO) a bug.
library(ggplot2)
library(grid)
#plot with factors
hod <- data.frame(h = c(1:24,1:24,1:24), mean = 1:(24*3) + runif(24*3, 0, 5),ci = runif(24*3, 0, 2), t = c(rep("a",24),rep("b",24),rep("c",24)))
pd <- position_dodge(0.3)
dayplot <- ggplot(hod, aes(x=h, y=mean, colour=as.factor(t),group=as.factor(t))) +
geom_line(position=pd, size=1) +
geom_errorbar(aes(ymin=mean-ci, ymax=mean+ci),
width=1,
size=0.5,
position=pd) +
geom_point(position=pd, shape=21, size=1, fill="white") +
scale_x_continuous(limits=c(-0.5,23.5),
breaks=c(0:8*3),
labels=ifelse(
c(0:8*3) < 10,
paste('0',c(0:8*3),':00',sep=''),
paste(c(0:8*3),':00',sep='')
)
) +
xlab("Hour of day") +
theme_minimal() +
theme(plot.margin = unit(c(1,0,1,1), "cm"),
axis.title.x = element_text(vjust=-1),
axis.title.y = element_text(angle=90, vjust=0),
legend.margin = unit(c(0), "cm"),
legend.key.height = unit(c(0.9), "cm"),
panel.grid.major = element_line(colour=rgb(0.87,0.87,0.87)),
panel.grid.minor = element_blank(),
plot.background = element_rect(fill = rgb(0.97,0.97,0.97), linetype=0)
)
print(dayplot)
#plot without factors
hod <- data.frame(h = c(1:24,1:24,1:24), mean = 1:(24) + runif(24, 0, 5),ci = runif(24, 0, 2))
pd <- position_dodge(0.3)
dayplot <- ggplot(hod, aes(x=h, y=mean)) +
geom_line(position=pd, size=1) +
geom_errorbar(aes(ymin=mean-ci, ymax=mean+ci),
width=1,
size=0.5,
position=pd) +
geom_point(position=pd, shape=21, size=1, fill="white") +
scale_x_continuous(limits=c(-0.5,23.5),
breaks=c(0:8*3),
labels=ifelse(
c(0:8*3) < 10,
paste('0',c(0:8*3),':00',sep=''),
paste(c(0:8*3),':00',sep='')
)
) +
xlab("Hour of day") +
theme_minimal() +
theme(plot.margin = unit(c(1,0,1,1), "cm"),
axis.title.x = element_text(vjust=-1),
axis.title.y = element_text(angle=90, vjust=0),
legend.margin = unit(c(0), "cm"),
legend.key.height = unit(c(0.9), "cm"),
panel.grid.major = element_line(colour=rgb(0.87,0.87,0.87)),
panel.grid.minor = element_blank(),
plot.background = element_rect(fill = rgb(0.97,0.97,0.97), linetype=0)
)
print(dayplot)

Answer 2

I have managed to solve a similar issue. In my case I wanted to set both horizontal and vertical errorbar heads to the same size - regardless of the aspect ratio of the plot.
Based on the original posted code:
f <- ggplot_build(dayplot)
f$plot$layers[[5]]$geom_params$width <- 0.02 * diff(f$layout$panel_params[[1]]$x.range)
f$plot$layers[[6]]$geom_params$height <- 0.02 * diff(f$layout$panel_params[[1]]$y.range)
dayplot <- f$plot
This will set the errorbar head to 2% of the axis range. Maybe could solve your issue.