ggplot legend issue w/ geom_point and geom_text

Question

I am trying to use geom_point to illustrate the count of my data. I would also like to annotate a few of the points in my graph with geom_text. When I add the call to geom_text, it appears that it is plotting something underneath the points in the legend. I've tried reversing the order of the layers to no avail. I can't wrap my head around why it is doing this. Has anyone seen this before?

set.seed(42)
df <- data.frame(x = 1:10
    , y = 1:10
    , label = sample(LETTERS,10, replace = TRUE)
    , count = sample(1:300, 10, replace = FALSE)
)

p <- ggplot(data = df, aes(x = x, y = y, size = count)) + geom_point()
p + geom_text(aes(label = label, size = 150, vjust = 2))

Answer 1

This happened to me all the time. The trick is knowing that aes() maps data to aesthetics. If there's no data to map (e.g., if you have a single value that you determine), there's no reason to use aes(). I believe that only things inside of an aes() will show up in your legend.

Furthermore, when you specify mappings inside of ggplot(aes()), those mappings apply to every subsequent layer. That's good for your x and y, since both geom_point and geom_text use them. That's bad for size = count, as that only applies to the points.

So these are my two rules to prevent this kind of thing:

1. Only put data-based mappings inside of aes(). If the argument is taking a single pre-determined value, pass it to the layer outside of aes().
2. Map data only for those layers that will use it. Corollary: only map data inside of ggplot(aes()) if you're confident that every subsequent layer will use it. Otherwise, map it at the layer level.

So I would plot this thusly:

p <- ggplot(data = df, aes(x = x, y = y)) + geom_point(aes(size = count)) 
p + geom_text(aes(label = label), size = 4, vjust = 2)