test for cache dependencies
source: https://d.cosx.org/d/418610-418610
# 1st time a:
# gg1 = ggplot(mtcars, aes(x= mpg, y= cyl)) +geom_point()
# 2nd time, gg1 changed
gg1 = ggplot(mtcars, aes(x= mpg, y= cyl)) +geom_boxplot()
gg2 = ggplot(mtcars, aes(x= wt, y= vs)) +geom_point()
grid.arrange(gg1,gg2)
grid.arrange(gg1,gg2)
## Warning: Continuous x aesthetic -- did you forget aes(group=...)?