[R] Why do histogram bars vary their width?

[James Rome]

histogram(~(Arrival4) | as.factor(Hour), type="count",
breaks=16,ylab="Arrival Count",
  xlab="Arrival Rate/4",main="Friday EWR A22R D22L Configiration",
  layout=c(6,4), par.strip.text=list(cex=0.7))

Why do I get plots with different bar widths? See attached.

Thanks,
Jim Rome
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