guide_legend and ggplot2, format nrow - r

Question

I am trying to format an over-long legend on a ggplot so that there is a maximum no. of rows. I've read all the documentation that I could find, especially this: http://docs.ggplot2.org/0.9.3.1/guide_legend.html but for some reason, the legend will not format.
I've given a reproducible sample below using the quakes dataset, and converted the column stations to character so that they plot individually (otherwise, they seem to plot as groups).
plotquakes <- function(magreq) {
library(ggplot2)
magdata <- subset(quakes, mag > magreq)
magdata$stations <- as.character(magdata$stations)
g <- ggplot(magdata, aes (x = lat, y = long))
g + geom_point(aes(alpha = stations), fill = "black", pch=21, size = 6) +
labs(x = "Latitude", y = "Longitude") +
geom_vline(xintercept = 0, col = "red") +
geom_hline(yintercept = 0, col = "red") +
guides(col = guide_legend(nrow = 16))
}
plotquakes(5)
And what I get is this:
whereas I would like to have a maximum of 16 data fields per column in the legend.

Answer 1

You are changing the wrong guide.
plotquakes <- function(magreq) {
library(ggplot2)
magdata <- subset(quakes, mag > magreq)
magdata$stations <- as.character(magdata$stations)
g <- ggplot(magdata, aes (x = lat, y = long))
g + geom_point(aes(alpha = stations), fill = "black", pch=21, size = 6) +
labs(x = "Latitude", y = "Longitude") +
geom_vline(xintercept = 0, col = "red") +
geom_hline(yintercept = 0, col = "red") +
guides(alpha = guide_legend(nrow = 16)) #note it's alpha not col
}
plotquakes(5)