library(ggplot2)
theme_set(theme_classic())

# Histogram on a Continuous (Numeric) Variable
g <- ggplot(mpg, aes(displ)) + scale_fill_brewer(palette = "Spectral")

g + geom_histogram(aes(fill=class), 
	bins=5,		# Il numero dei bin e' fissato  
	col="black", 
	size=.1) +   
	labs(title="Histogram with Fixed Bins", 
             subtitle="Engine Displacement across Vehicle Classes")