title: Warning: Encountered two children with the same key
body: <p>I am using nivo line component to draw a stacked line graph.  I am getting this error:</p>
  
  <pre><code>Warning: Encountered two children with the same key, `L1.87.83999837239584`. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version
  </code></pre>
  
  <p>What keys and children is it talking about?  Is it remotely possible that each data point on the graph is a distinct React element which is part of some collection and must be uniquely identified in "react sense"?</p>