r - ggplot2 change colours how

Question

I want to do a ggplot2 scatter plot

    scores <- data.frame( SampleID = rep(LETTERS[1:3], 5), PC1 = rnorm(15), PC2 = rnorm(15) )
library( ggplot2 )
ggplot( scores, aes( x = PC1, y = PC2, colour = SampleID ) ) +
  geom_point()

this code colours the data points in a gradient, so that thez are often not really distinguishable. I saw that

http://docs.ggplot2.org/current/geom_point.html

uses

geom_point(aes(colour = factor(cyl)))

for colouring but if I enter

ggplot( scores, aes( x = PC1, y = PC2, colour = SampleID ) ) +
      geom_point(aes(colour = factor(cyl)))

I get an error message

 in factor(cyl) : object 'cyl' not found

can somebody tell me how I can colour the scatter graph with either not gradient colours OR different symbols?

Answer 1

scale_color_manual let's you pick the colors used.

ggplot( scores, aes( x = PC1, y = PC2, colour = SampleID ) ) +
    geom_point() +
    scale_color_manual(values = c("red", "black", "dodgerblue2"))

The cyl in the example refers to the cyl column of the mtcars dataset used in the example. If you would rather use shapes then colors, don't use the colour aesthetic, use the shape aesthetic instead.

ggplot( scores, aes( x = PC1, y = PC2, shape = SampleID ) ) +
    geom_point()

If you want to choose shapes (using usual R pch codes), then use scale_shape_manual.