title: I want to repeat plotting for each section
body: <p>I am trying to repeat plotting in R
  my main command is </p>
  
  <pre><code>data&lt;-read.csv(file.choose(),header=TRUE)
  t=data[,1]
  PCI=data[,2]
  plot(t,PCI,xlim=c(0,30))
  boxplot(t,PCI,xlim=c(0,30))
  # some starting values
  alpha=1
  betha=1
  theta=1
  # do the fit
  fit = nls(ydata ~ alpha-beta*exp((-theta*(xdata^gama))),      start=list(alpha=80,beta=15,theta=15,gama=-2))
  
  ydata=data[,2]
  xdata=data[,1]
  new = data.frame(xdata = seq(min(xdata),max(xdata),len=200))
  lines(new$xdata,predict(fit,newdata=new))
  resid(fit)
  qqnorm(resid(fit))
  
  fitted(fit)
  resid(fit)
  xdata=fitted(fit) 
  ydata=resid(fit)
  plot(xdata,ydata,xlab="fitted values",ylab="Residuals")
  abline(0, 0)
  </code></pre>
  
  <p>My first column is  number of section, my second column is t=x and my third column is PCI=y. I want to repeat my plotting command for each section individually.but I think I can not use loop since the number of data in each section is not equal.
  I would really appreciate your help since i am new in R. </p>
  
  <pre><code>SecNum  t   PCI
  961 1   94.84
  961 2   93.04
  961 3   91.69
  961 11  80.47
  961 12  79.26
  961 13  77
  962 1   90.46
  962 2   90.01
  962 3   86.88
  962 4   86.36
  962 5   84.56
  962 6   85.11
  963 1   91.33
  963 2   90.7
  963 3   86.46
  963 4   88.47
  963 5   81.07
  963 6   84.07
  963 7   82.55
  963 8   73.58
  963 9   71.85
  963 10  83.8
  963 11  82.16
  </code></pre>