How do I plot charts with nested categories axes?

Question

I\'m plotting data which have both a category and a sub-category (see the example data below) and I\'d like to display these with them nested (this example was created in Ex

Answer 1

Untested, but try:

ggplot(data=df, aes(x=second.cat, y=value)) + geom_point() + facet_grid(~ main.cat, scales = 'free')

Albeit, the width of each main.cat would be the same, and the label of same can only be positioned at top.