解决HighStock点太多，显示tooltip时NAN

groupPixelWidth: Number

The approximate pixel width of each group. If for example a series with 30 points is displayed over a 600 pixel wide plot area, no grouping is performed. If however the series contains so many points that the spacing is less than the groupPixelWidth, Highcharts will try to group it into appropriate groups so that each is more or less two pixels wide. If multiple series with different group pixel widths are drawn on the same x axis, all series will take the greatest width. For example, line series have 2px default group width, while column series have 10px. If combined, both the line and the column will have 10px by default. Defaults to2. Defaults to2.

官网对于此属性的解释，将此值改小即可解决此问题（我设置的是0.1）。

plotOptions.series.dataGrouping.groupPixelWidth