r - Color one point and add an annotation in ggplot2?

Question

I have a dataframe a with three columns :

GeneName, Index1, Index2

I draw a scatterplot like this

ggplot(a, aes(log10(Index1+1), Index2)) +geom_point(alpha=1/5)

Then I want to color a point whose GeneName is "G1" and add a text box near that point, what might be the easiest way to do it?

Answer 1

You could create a subset containing just that point and then add it to the plot:

# create the subset
g1 <- subset(a, GeneName == "G1")

# plot the data
ggplot(a, aes(log10(Index1+1), Index2)) + geom_point(alpha=1/5) +  # this is the base plot
  geom_point(data=g1, colour="red") +  # this adds a red point
  geom_text(data=g1, label="G1", vjust=1) # this adds a label for the red point

NOTE: Since everyone keeps up-voting this question, I thought I would make it easier to read.