Control the size of points in an R scatterplot?

128
votes

In R, the plot() function takes a pch argument that controls the appearance of the points in the plot. I'm making scatterplots with tens of thousands of points and prefer a small, but not too small dot. Basically, I find pch='.' to be too small, but pch=19 to be too fat. Is there something in the middle or some way to scale the dots down somehow?

3
rplotscatter-plot
Rather than making dots smaller, is there a way to just plot say 100 random data points would would be easier to see than 1000? - user2363642
For many small points, using alpha (transparency) can make a scatter plot much more informative than just using fewer points to plot it. - user334911
I suggest not to use pch='.', it will be off center. Source: I used to use pch='.' often - geneorama
@geneorama Do you mean using ยท instead of .? - nanaki
@nanaki more or less, but oddly your symbol shows up as tiny boxes when I use it in R. The answers below do a good job at explaining the right approach. - geneorama

3 Answers

111
votes

Try the cex argument:

?par

  • cex
    A numerical value giving the amount by which plotting text and symbols should be magnified relative to the default. Note that some graphics functions such as plot.default have an argument of this name which multiplies this graphical parameter, and some functions such as points accept a vector of values which are recycled. Other uses will take just the first value if a vector of length greater than one is supplied.
98
votes

pch=20 returns a symbol sized between "." and 19.

It's a filled symbol (which is probably what you want).

Aside from that, even the base graphics system in R allows a user fine-grained control over symbol size, color, and shape. E.g.,

dfx = data.frame(ev1=1:10, ev2=sample(10:99, 10), ev3=10:1)

with(dfx, symbols(x=ev1, y=ev2, circles=ev3, inches=1/3,
                  ann=F, bg="steelblue2", fg=NULL))

Graph example

25
votes

As rcs stated, cex will do the job in base graphics package. I reckon that you're not willing to do your graph in ggplot2 but if you do, there's a size aesthetic attribute, that you can easily control (ggplot2 has user-friendly function arguments: instead of typing cex (character expansion), in ggplot2 you can type e.g. size = 2 and you'll get 2mm point).

Here's the example:

### base graphics ###
plot(mpg ~ hp, data = mtcars, pch = 16, cex = .9)

### ggplot2 ###
# with qplot()
qplot(mpg, hp, data = mtcars, size = I(2))
# or with ggplot() + geom_point()
ggplot(mtcars, aes(mpg, hp), size = 2) + geom_point()
# or another solution:
ggplot(mtcars, aes(mpg, hp)) + geom_point(size = 2)