r ggplot geom_point change color

Question

I am running the code and it works

ggplot(data_df, aes(x= RR, y= PPW, col = year)) + 
  geom_point(size = 3, alpha=0.6)

Now I am trying to put

Answer 1

You're missing two key points about how ggplot manages aesthetics:

1. Each geom_* layer will inherit the aes settings from the parent ggplot call, unless you manually override it. So in you first example, the 3rd geom_point inherits the x and y values from ggplot, not the "mean" layer above it, and thus renders a red point on top of each colored point.

2. Values in the aes are applied to a scale, not used as is. So when you put color = 'red' in the aes in your second example, you aren't making the points red, you're saying that color should be determined by a categorical variable (which here is a length 1 vector consisting of the word 'red') based on the scale_color_*. You can either add a scale_color_manual and set 'red' = 'red', so that value renders the desired color, or move the color= outside the aes, so it is interpreted as is (and will make all points in that layer red).

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With those points in mind, doing what you want is as simple as moving the color outside the aes:

ggplot(data_df, aes(x= RR, y= PPW, col = year))) + 
    geom_point(size = 3, alpha=0.6) + 
    geom_point(data=data_df, aes(x=mean(RR), y=mean(PPW)), color="red")