Use double.NaN when you don't know a value in a line series, that point will be undefined and will be ignored when the series is drawn.
其只能用在线形图（LineSeries）中，在其他类型图表中运用时会出错。
Use double.NaN when you don't know a value in a line series, that point will be undefined and will be ignored when the series is drawn.
其只能用在线形图（LineSeries）中，在其他类型图表中运用时会出错。
