Empty or Null dataPoints are points with no Y value. These are used in cases where values are not available for a specific x value. In cases of charts that require axis (line, area, bar, etc), null dataPoints introduce an empty space or gap and in case of other chart types (pie, doughnut) only labels show up at corresponding position. Empty dataPoints are sometimes referred to as missing dataPoints. Below are some of the examples for empty / null dataPoints.
Note that the line breaks at the point where empty data point is present and they don’t appear in the toolTip
Note that the line breaks are replaced with dotted line at the point where empty data point is present.
Try changing the chart type to any of the other supported chart type to see how they behave.