The Cumulative Sum tool sums (cumulates) all the previous values with the current value in a visible range.
For the following data:
[10, 8, 15, 20, 8, 15] the Cumulative Sum returns
[10, 18, 33, 53, 61, 76] (see the graph below):
cumulative can be enabled in the chart's options using the series.cumulative property or enabled/disabled by the series.setCumulative() method or on all the series belonging to a specific y-axis by the yAxis.setCumulative() method.