ggplot2: how to produce smaller points

Question

I have a large dataset that I am plotting using a scatter plot. These points have a unique combination of x,y and therefore they don't overlap, but some of them are very close to each other therefore I'm plotitng them with small size.

1- How to produce smaller point symbols (smaller size ) so that the areas are proportional. In this example, the last point does not have an area proportional to the size . I was expecting it 10 smaller than the middle one eg:

df <- data.frame(c1 = 1:3, c2 = c(1,1,1))
ggplot(df) + geom_point(aes(x= c1, y = c2), size = c(1, 0.1, 0.01)) 

2- How does the size in ggplot2 matches the R graphics cex argument eg: plot(df$c2 ~ df$c1, cex = c(1, 0.1, 0.01)) . Thanks

Answer 1

You can try

geom_point(shape = ".") 

this will make the point 1 pixel in size.

This is from page 70 of ggplot2 second edition by H Wickham

Answer 2

There is a size = argument to geom_point , but you either specify a size for all points:

+ geom_point(size = 0.5)

Or you map the size to one of the columns in your data using aes :

+ geom_point(aes(size = c2))

In the latter case, you can control the range of sizes using scale_size_continuous . The default is min = 1, max = 6. To get eg min = 2, max = 8:

+ geom_point(aes(size = c2)) + scale_size_continuous(range = c(2, 8))

• Note that the "ggplot2 way" is to map data to geoms, not to assign values to each observation
• and no, size here is different to cex