
How to make small multiple charts with ggplot2 (facet_grid) - SharpSightLabs
https://www.sharpsightlabs.com/blog/facet_grid/
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black-tea
It took me a while to understand that facets are a way of displaying another
dimension. Facet wrap lets you display one more dimension and grid lets you
display two. I've produced some great plots with both.

