Create ggplot2-based visualisations from model objects.
This generic is defined in ackwards so that autoplot.ackwards() is
available without requiring library(ggplot2). When ggplot2 is also
loaded its own autoplot generic takes over in the search path, but S3
dispatch still finds autoplot.ackwards correctly via either route.
Defining our own generic is intentional – it avoids putting ggplot2 in
Imports while keeping the autoplot(x) ergonomic without a library() call.