ggplot(hw, aes(x = ageYear, y = heightIn, shape = sex, fill = weightGroup)) +
geom_point(size=2.5)+scale_shape_manual(values=c(21,24)) + scale_fill_manual(values = c(NA, "black"), guide = guide_legend(override.aes = list(shape = 21)))

请问guide = guide_legend(override.aes = list(shape = 21)）这行是什么意思？