title: Using plotly's hoverinfo: "x+y" with mode: "lines" shows only y values
body: <p>I'm trying to plot a line graph and want the hover info to show the value of <em>x</em> and the value of <em>y</em>.</p>
  
  <p>My trace has the following properties (among others):</p>
  
  <pre><code>  type: 'scatter',
    mode: 'lines',
    hoverinfo: "x+y"
  </code></pre>
  
  <p>The problem is that the hover shows only the <em>y</em> value, although I have selected <code>hoverinfo: "x+y"</code>.</p>
  
  <p>Note that I would like to show the <em>x</em> values without converting them to <code>text: [x-values]</code>, which looks a bit hacky.</p>
  
  <p>How can this be done? </p>