title: Create a secondary y axis for lines graph with ggvis R
body: <p>This is what i have so far with the ggvis package in R.</p>
  
  <pre><code>    mtcars %&gt;% ggvis(x = ~disp) %&gt;% 
      layer_lines(y = ~wt, stroke := "red") %&gt;%
      layer_lines(y = ~mpg) %&gt;%
      add_axis("y", orient = "left", title = "Weight (lb/1000)") %&gt;%
      add_axis("y", orient = "right", title= "Miles/(US) gallon") %&gt;%
      add_axis("x", title = "Displacement (cu.in.)")
  </code></pre>
  
  <p>I cannot get the left Y axis to represent the wt scale data.</p>
  
  <p>This outputs:</p>
  
  <p><img src="https://i.stack.imgur.com/yCj9X.jpg" alt="enter image description here"></p>