adding a vertical line with a different intercept to each facet of a histogram in ggplot2

[Bonnie Dixon]

Dear D-RUG folks;

I could use help with a graphic I would like to make.  

I made a set of faceted histograms showing the distributions of four groups of observations:

(TST.hist <- ggplot(Hunger, aes(x = TSThrs, group = BedtimeHunger)) + geom_histogram()) + facet_wrap(~ BedtimeHunger, nrow = 4) + theme_bw()

Next, I would like to add a line to each facet showing the mean for that group of observations.  I tried this:

(TST.hist <- ggplot(Hunger, aes(x = TSThrs, group = BedtimeHunger)) + geom_histogram()) + facet_wrap(~ BedtimeHunger, nrow = 4) + theme_bw() + geom_vline(xintercept = mean(Hunger$TSThrs, na.rm = TRUE), col = "red")

But it drew a line at the grand mean, not the mean of each group (as shown in the attachment).

Then I tried this:

(TST.hist <- ggplot(Hunger, aes(x = TSThrs, group = BedtimeHunger)) + geom_histogram() + theme_bw() + geom_vline(data=aggregate(Hunger$TSThrs, list(Hunger$BedtimeHunger), mean, na.rm = TRUE), mapping=aes(xintercept = x), color="red")) + facet_wrap(~ BedtimeHunger, nrow = 4)

But it drew four lines on each facet showing the means of all four groups (as shown in the second attachment).

Does anyone have an idea for how to get one line on each facet showing the mean for that group?

Thanks a bunch for your help!

Bonnie

Bonnie M. Dixon, MCP
Doctoral Candidate in Nutritional Biology

Foods for Health Institute

1221 RMI South
University of California, Davis

One Shields Avenue, Davis, CA 95616

530-908-7687, bmd...@ucdavis.edu

ffhi.ucdavis.edu/people/directory/bmdixon

[Noam Ross]

There's an example of this in the online documentation at http://docs.ggplot2.org/0.9.3.1/geom_vline.html. You need to create a second data.frame holding the means and organized by the facet variable. So, using plyr::ddply:

library(plyr)
TST.hist <- ggplot(Hunger, aes(x = TSThrs, group = BedtimeHunger)) + geom_histogram()) + facet_wrap(~ BedtimeHunger, nrow = 4) + theme_bw()
TST.hist.vlinedata <- ddply(Hunger, "BedtimeHunger", summarize, BedMean=mean(TSThrs))
TST.hist <- TST.hist + geom_vline(aes(xintercept=BedMean),  TST.hist.vlinedata, col="red")

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[Bonnie Dixon]

That worked.  Thanks, Noam!  Good to know about that online documentation for ggplot too.

Bonnie