title: How to add title at the top and in the middle of two graph in R
body: <p>R code :</p>
  
  <pre><code>windows(width=9, height=6)
  par(mfrow=c(1,2))
  
  curve(dweibull(x,1,.5),from=0,to=4,col="blue",xlab="t",ylab="f(t)",main="Probability density function")
  curve(dweibull(x,1.5,1),add=TRUE,col="red")
  curve(dweibull(x,3,1),add=T,col="green")
  
  text(3.5,2,expression(beta==.5),col="blue")
  text(3.5,1.88,expression(beta==1.5),col="red")
  text(3.5,1.76,expression(beta==3),col="green")
  
  
   ht=function(x,lambda,beta){
      h=lambda*beta*(lambda*x)^(beta-1)
      return(h)
   }
  
  
  
  curve(ht(x,1,.5),xlim=c(0,4),ylim=c(0,4),xlab="t",ylab="f(t)",col="blue",main="Hazard function")
  curve(ht(x,1,1.5),add=T,col="red")
  curve(ht(x,1,3),add=T,col="green")
  
  text(3.5,4,expression(beta==.5),col="blue")
  text(3.5,3.85,expression(beta==1.5),col="red")
  text(3.5,3.70,expression(beta==3),col="green")
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/lE0fI.jpg" alt=""></p>
  
  <p>I want to write "Weibull distribution" at the top and in the middle of both graph, like the following :</p>
  
  <p><img src="https://i.stack.imgur.com/Y8dih.png" alt="enter image description here"></p>
  
  <p>How can i do that ?</p>