[R] Two-levels labels on x-axis?

[Marc Schwartz]

On Aug 22, 2011, at 12:14 PM, Sébastien Vigneau wrote:

> Hi,
> 
> I would like to draw a stacked bar chart with four bars (say "a", "b", "c",
> "d") . Two bars belong to group A and the two others to group B. Therefore,
> I would like to have, on the x-axis, a label for each bar and an additional
> label for each group, positioned underneath. To give an idea, the x-axis
> labels should look like this:
> |a|b|c|d|
> | A | B |
> 
> Do you know how I can generate such two-levels labels in R?
> 
> Thank you for your help!
> 
> Sebastien


See ?barplot and note in the Value section:

  If beside is true, use colMeans(mp) for the midpoints of each group of bars, see example.


Here is another example:

mat <- matrix(1:4, ncol = 2)

mp <- barplot(mat, beside = TRUE)

Groups <- c("A", "B")
Sub.Groups <- c("a", "b", "c", "d")

mtext(side = 1, line = 1, at = mp, text = Sub.Groups)
mtext(side = 1, line = 3, at = colMeans(mp), text = Groups)


HTH,

Marc Schwartz