Every analysis function in movecost 3.0 returns an object that stores its
results; visualisation is performed afterwards, on demand, by the
plot() methods documented here. Each method returns a
ggplot object: printed at the console it draws the map, assigned to
a variable it can be customised with standard ggplot2 syntax (additional
layers, themes, scales) and drawn any number of times - all without
re-running any computation. ggplot2::autoplot() methods are provided
as aliases.