copied from r-spatial/sf#88

HI, I tried point and line geometry with (the experimental) 'geom_sf' for plotting maps (in simple features format ):

library(sf)
library(maps)
library(ggplot2)

#points
data(meuse,package="sp")
meuse_sf <- st_as_sf(meuse, coords = c("x", "y"), crs = 28992, agr = "constant")

summary(meuse_sf)

meuse_sf %>% ggplot() + geom_sf(aes(size=cadmium))

this gives:

I am a bit confused by the behaviour of aes in geom_sf, with aes(size=cadmium) I expected varying size of dots, instead it gives me varying size of line thickness of the circles.
How do I change type and size of dots?

meuse_sf %>% ggplot() + geom_sf(aes(color=cadmium)) gives:

line color of the circles changes, I want a fill....

meuse_sf %>% ggplot() + geom_sf(aes(fill=cadmium))
this gives:

more confusing, I still want a fill.... :-), how can I do that ?