title: React Vis x can't be a string
body: <p>I am using React Vis to create a simple Graph. This is my code:</p>
  <pre><code>import React from 'react';
  import '../../../node_modules/react-vis/dist/style.css';
  import {XYPlot, VerticalBarSeries} from 'react-vis';
  
  const myData = [
    {x: 'A', y: 10},
    {x: 'B', y: 5},
    {x: 'C', y: 15}
  ]
  
  
  export const Chart = () =&gt; {
    return (
        &lt;XYPlot height={500} width={500}&gt;
          &lt;VerticalBarSeries data={myData} /&gt;
        &lt;/XYPlot&gt;
    )
  };
  
  export default Chart;
  </code></pre>
  <p>I receive this error message:
  <code>Received NaN for the `x` attribute. If this is expected, cast the value to a string.&quot;</code></p>
  <p>When I change the x values in myData to ints, it works, but I need them as strings. The documentation clearly states that this should work:
  <a href="https://i.stack.imgur.com/urOXW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/urOXW.png" alt="enter image description here" /></a></p>