R – What do hjust and vjust do when making a plot using ggplot

ggplot2plotr

Every time I make a plot using ggplot, I spend a little while trying different values for hjust and vjust in a line like

+ opts(axis.text.x = theme_text(hjust = 0.5))

to get the axis labels to line up where the axis labels almost touch the axis, and are flush against it (justified to the axis, so to speak). However, I don't really understand what's going on. Often, hjust = 0.5 gives such dramatically different results from hjust = 0.6, for example, that I haven't been able to figure it out just by playing around with different values.

Can anyone point me to a comprehensive explanation of how hjust and vjust options work?

Best Answer

The value of hjust and vjust are only defined between 0 and 1:

0 means left-justified
1 means right-justified

Source: ggplot2, Hadley Wickham, page 196

(Yes, I know that in most cases you can use it beyond this range, but don't expect it to behave in any specific way. This is outside spec.)

hjust controls horizontal justification and vjust controls vertical justification.

An example should make this clear:

td <- expand.grid(
    hjust=c(0, 0.5, 1),
    vjust=c(0, 0.5, 1),
    angle=c(0, 45, 90),
    text="text"
)

ggplot(td, aes(x=hjust, y=vjust)) + 
    geom_point() +
    geom_text(aes(label=text, angle=angle, hjust=hjust, vjust=vjust)) + 
    facet_grid(~angle) +
    scale_x_continuous(breaks=c(0, 0.5, 1), expand=c(0, 0.2)) +
    scale_y_continuous(breaks=c(0, 0.5, 1), expand=c(0, 0.2))

enter image description here

To understand what happens when you change the hjust in axis text, you need to understand that the horizontal alignment for axis text is defined in relation not to the x-axis, but to the entire plot (where this includes the y-axis text). (This is, in my view, unfortunate. It would be much more useful to have the alignment relative to the axis.)

DF <- data.frame(x=LETTERS[1:3],y=1:3)
p <- ggplot(DF, aes(x,y)) + geom_point() + 
    ylab("Very long label for y") +
    theme(axis.title.y=element_text(angle=0))


p1 <- p + theme(axis.title.x=element_text(hjust=0)) + xlab("X-axis at hjust=0")
p2 <- p + theme(axis.title.x=element_text(hjust=0.5)) + xlab("X-axis at hjust=0.5")
p3 <- p + theme(axis.title.x=element_text(hjust=1)) + xlab("X-axis at hjust=1")

library(ggExtra)
align.plots(p1, p2, p3)

enter image description here

To explore what happens with vjust aligment of axis labels:

DF <- data.frame(x=c("a\na","b","cdefghijk","l"),y=1:4)
p <- ggplot(DF, aes(x,y)) + geom_point()

p1 <- p + theme(axis.text.x=element_text(vjust=0, colour="red")) + 
        xlab("X-axis labels aligned with vjust=0")
p2 <- p + theme(axis.text.x=element_text(vjust=0.5, colour="red")) + 
        xlab("X-axis labels aligned with vjust=0.5")
p3 <- p + theme(axis.text.x=element_text(vjust=1, colour="red")) + 
        xlab("X-axis labels aligned with vjust=1")


library(ggExtra)
align.plots(p1, p2, p3)

enter image description here

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