title: Tooltip appears for all nivo charts except nivo/line
body: <p>I am using nivo charts and the tooltip is not appearing only for my line chart. I copy pasted the example here:</p>
  <p><a href="https://nivo.rocks/line/" rel="nofollow noreferrer">https://nivo.rocks/line/</a></p>
  <p>The chart appears fine with a crosshair but no tooltip appears. When I copy paste other chart examples they work perfectly with a tooltip appearing. I am not sure how to go about solving this as it only happens for the line chart.</p>