I have trouble making a filled / open circle plot by group.
My data looks like:
ggplot(data, aes(x=Gender, y=Body_weight))+
The aesthetic "fill" does not determine if the point is filled with colour or not, but fills it with a different colour for each gender. The way to do what you want is using "shape" and mapping the shapes to open and closed circle instead of the default circle/triangle you get. To do this you need to use
ggplot(data, aes(x=Gender, y=Body_weight)) + geom_point(aes(shape=Gender)) + scale_shape_manual(values = c(16, 21))
Have a look at http://sape.inf.usi.ch/quick-reference/ggplot2/shape for other shapes available