如何修改ggplot密度图的点类型？

Question

我想以灰色绘制5个密度的图（以打印我的图），这些密度非常接近。另外，如果我只是修改行的类型，还不清楚。因此，我想修改积分的类型。有人有解决办法吗？
目前，我将其绘制为：

# An example of data
df = data.frame(speed = c(80 + rnorm(n = 10, sd = 7),
                          100 + rnorm(n = 10, sd = 7),
                          90 + rnorm(n = 10, sd = 7)),
                vehicule = c(rep("bus",10), rep("moto",10), rep("car",10)))

# Graph with different types of lines but not different types of points
ggplot(df, aes(speed, linetype = vehicule)) +
geom_density(alpha = 0.4)

或：

# Graph with different type of points but not density
ggplot(df, aes(speed, y = vehicule, linetype = vehicule)) +
geom_point(aes(shape=vehicule)) +
geom_density(alpha = 0.4) + 
scale_color_grey() +
scale_fill_grey()