# Separate histogram for each voice part (Figure 1.2 from Cleveland)
histogram(~ height | voice.part,
          data = singer,
          aspect = 1,
          layout = c(2, 4), 
          nint = 15,
          xlab = "Height (inches)")
# Quantile-Quantile plot (Figure 2.11 from Cleveland)
qqmath(~ height | voice.part,
       data = singer,
       aspect = 1, 
       layout = c(2,4),
       prepanel = prepanel.qqmathline,
       panel = function(x, ...) {
         panel.grid()
         panel.qqmathline(x, ...)
         panel.qqmath(x, ..., grid = FALSE)
       },
       xlab = "Unit Normal Quantile",
       ylab="Height (inches)")
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