title: Overlaying two/more graphs using R
body: <p>I have been referring stackoverflow posts and came up with query over a gglot2 features.</p>
  
  <p>The data set used for code is as follows.</p>
  
  <pre><code>TimeStamp   data_1      data_2      data_3      
  15:11:37.474    99.791028   0.312498    99.47853    
  15:16:22.373    99.791028   0.729162    99.061866   
  15:21:37.424    99.791028   0.104166    99.686862   
  15:31:52.475    88.02027            90.520254   
  15:42:07.157    99.99936    0.208332    99.791028   
  15:43:22.279    99.99936    0.52083     99.47853    
  15:45:37.673    99.686862   0       99.686862   
  15:52:52.872    99.686862   0.729162    98.9577 
  
  p1&lt;- ggplot(df, aes(TimeStamp, data_1,group=1)) + geom_point() + geom_point(data = df[df$data_1 &gt; 80,], pch = 21, fill = NA, size = 4, colour = "red", stroke = 1) + geom_line()
  p2&lt;-ggplot(df, aes(TimeStamp, data_3,group=1)) + geom_point() + geom_point(data = df[df$data_3 &gt; 70,], pch = 21, fill = NA, size = 4, colour = "blue", stroke = 1) + geom_line()
  </code></pre>
  
  <p>I am trying to overlay P1 and P2 into same X and Y axis using <code>print(p1+p2).</code>
  But, came up with below error.</p>
  
  <pre><code>Error in p + o : non-numeric argument to binary operator
  In addition: Warning message:
  Incompatible methods ("+.gg", "Ops.data.frame") for "+" 
  </code></pre>
  
  <p>what these graphs will do is; it will circle certain data points based on conditions and display two graphs in same region.</p>