关于在直方图上添加正态密度曲线的问题...
R语言代码如下：
x <- mtcars$mpg

h <- hist(x,breaks = 12,col ="red",xlab = "Miles Per Gallon",main = "Histogram with normal curve and box")


xfit <- seq(min(x),max(x),length = 40)
yfit <- dnorm(xfit,mean = mean(x),sd = sd(x))
yfit <- yfit*diff(h$mids[1:2])*length(x)

lines(xfit,yfit,col = "BLUE",lwd = 2)
就是这一步yfit <- yfit*diff(h$mids[1:2])*length(x)
不懂，为什么要这么转化，而且用的是什么公式呢？求大神指点