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

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