在散点图中添加图例

Question

如何将图例添加到以下散点图

library(UsingR) 
data(galton) 

freqData <- as.data.frame(table(galton$child, galton$parent))
names(freqData) <- c("child", "parent", "freq")
plot(as.numeric(as.vector(freqData$parent)), 
      as.numeric(as.vector(freqData$child)),
      pch = 21, col = "black", bg = "lightblue",
      cex = .15 * freqData$freq, 
      xlab = "parent", ylab = "child")