title: for loop plot in list of data frame
body: <p>I want to make multiple plot. </p>
  
  <p>So I split data frame, and tried to ggplot.</p>
  
  <pre><code>linkid &lt;- unique (orgin_dat$stdLinkId)
  temp &lt;- split(orgin_dat, orgin_dat$stdLinkId)
  
  p1 &lt;-  ggplot(temp$`1550007100`, aes(x=datetime, y=linkSpeed)) +
    geom_line() +
    ggtitle(linkid[i])
  ## it works
  
  p1 &lt;-  ggplot(temp$linkid[1], aes(x=datetime, y=linkSpeed)) +
    geom_line() +
    ggtitle(linkid[i])
  ## it doesn't work
  </code></pre>
  
  <p>My goal is</p>
  
  <pre><code>for(i in 1:50){
    assign(paste(p, i, sep=""), 
    ggplot(temp$linkid[i], aes(x=datetime, y=linkSpeed)) +
      geom_line() +
      ggtitle(linkid[i]))
  }
  ## it doesn't work
  </code></pre>
  
  <p>Thanks.</p>