title: Add a normal distribution line in histogram
body: <p>I've met a weird problem that I can't figure it out totally.
  I'm supposed to add a normal distribution line upon a histogram.I input every step's code but after typing lines function there's no response. I don't know what's wrong.Hope anyone help me!
  MY code are:</p>
  
  <pre><code>grades&lt;-mydata$Exam1
  hist(grades,breaks=20,freq=T) #A correct histogram comes out.
  mean(grades,na.rm=T) #there is NA in the column so I remove it when calculating mean.
  [1] 75.15278
  
  sd(grades,na.rm=T)  
  [1] 16.97443 
  
  x&lt;-seq(0,100,0.01) 
  y&lt;-dnorm(x,mean=mean(grades,na.rm=T),sd=sd(grades,na.rm=T))
  lines(x,y)#and there's no response!no line showed up!
  </code></pre>
  
  <p>Is anything wrong with my code?
  Thanks for your help!</p>