show how to make the scale can be automatically found by ggplot2.
most of the code is copied from:
https://stackoverflow.com/questions/49961135/how-to-tell-ggplot2-to-use-an-user-created-scale-for-a-new-aesthetic
reference: the code in ggplot2/R/scale-type.R
Based on the code in ggplot2/R/scale-type.R, there should be a scale named scale_mag_continuous in the parent environment of find_scale function. Then, this scale can be find automatically.
library(autoscale)
geo <- tibble(lon = 1:10, lat = 1:10, mag = 1:10, angle = 1:10)
ggplot(geo, aes(lon, lat)) +
geom_arrow(aes(mag = mag, angle = angle))
ggplot(geo, aes(lon, lat)) +
geom_arrow(aes(mag = mag, angle = angle)) +
scale_mag_continuous()