Including a Legend in a Scatterplot where size of plot indicates frequency

Question

I am trying to include a legend for a scatterplot where size of plot indicates number of pairings

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 = .10 * freqData$freq, 
      xlab = "parent", ylab = "child")
legend("bottomright","(freqData)",pch=21, title="freqData")

Answer 1

Changing the size of points in the legend can be done by passing a vector of pt.cex values to legend() . The following code was used to generate the sample plot. The example uses a square root of the frequency so that the point area is proportional to the count in that pairing.

# historical data
library('HistData')

# Galton Data
rawData <- Galton

# making a set of unique parings and counting frequency
freqData <- unique(rawData)
freqData$count <- NA

for(i in 1:nrow(freqData)){
  freqData$count[i] <- length(intersect(which(rawData$parent %in% freqData$parent[i]),which(rawData$chil %in% freqData$child[i])))
}

# making plots
plot(freqData$parent
     ,freqData$child
     ,pch=19 # plot symbol
     ,cex=0.1*sqrt(freqData$count)) # point expansion
# adding legend
legend('bottomright' # location
       ,legend=c(1,5,10,15,20,25,30,35) # entries
       ,title='count' # title
       ,pt.cex=0.1*sqrt(c(1,5,10,15,20,25,30,35)) # point expansion
       ,pch=19 # plot symbol
       ,ncol=2 # number of columns
       )