Control the size of points in an R scatterplot?

Try the cex argument:

?par

• cex
  A numerical value giving theamount by which plotting text andsymbols should be magnified relativeto the default. Note that somegraphics functions such asplot.default have an argument of thisname which multiplies this graphicalparameter, and some functions such aspoints accept a vector of valueswhich are recycled. Other uses willtake just the first value if a vectorof length greater than one issupplied.

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))

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)