A console error when mousing over Trend chart

[gregallensworth]

Noted as I was working on other things (#62) but before work had started, so not related...

• Bring up a Trend chart. Any trend chart.
• Move the mouse over the chart line, and see the tooltip. Additionally, this message appears in the console:

  Uncaught TypeError: Cannot read property 'year_month' of undefined
  at lookUpDatum (LineChart.jsx?79cf:195)
  at SVGRectElement.handleMouseMove (LineChart.jsx?79cf:226)
  at SVGRectElement.eval (d3.js?1499:934)

This seems to happen several times as the mouse moves.

[gregallensworth]

More diagnostics: At this time it's very early December, so December 2017 doesn't have data.

This is the missing datum being mentioned in LineChart.jsx lookUpDatum()