title: Plot with ggplot a graph with two y scales
body: <p>So I am trying to compile several dataframe in one graph. For my x value I have the number of days, and for y I have the frequency of something happening. 
  My problem is that one of the dataframe (df3) ranges from 0 to 3, and the others from 0 to 50. </p>
  
  <p>Dummy dataframes:</p>
  
  <pre><code>day&lt;-c(1,2,3)
  b&lt;-c(23,44,22)
  c&lt;-c(12,35,49)
  d&lt;-c(1,1,3)
  
  df1&lt;-data.frame(day,b)
  df2&lt;-data.frame(day,c)
  df3&lt;-data.frame(day,d)
  
  
  ggplot()+ 
    geom_line(data=df1, aes(x=day, y=df1$`b`), color="red") +
    geom_line(data=df2, aes(x=day, y=df2$`c` ), color="green")+
    geom_line(data=df3, aes(x=day, y=df3$`d` ), color="blue")+
    labs(x="Days", y="Number of occurrences")
  </code></pre>
  
  <p>This works fine, but I want to creat a diferent scale for df1 and df2, and another to df3. </p>
  
  <p>UPDATE: </p>
  
  <p>I am trying this, but it overwrites the previous scale: </p>
  
  <pre><code>d&lt;-ggplot()+ 
    geom_line(data=df1, aes(x=day, y=df1$`a`), color="red") +
    geom_line(data=df2, aes(x=day, y=df2$`b` ), color="green")+
    scale_y_continuous(limits=c(0, 50))+
    labs(x="Days", y="Number of occurrences")
  
  d+geom_line(data=df3, aes(x=day, y=df3$`c` ), color="blue")+
    scale_y_continuous(limits=c(0, 3))
  </code></pre>