By default, the area chart draws the series on top of one another. You can stack them atop one another instead, so that the data values at each x-value are summed. In an area chart, the value for each series will always be stacked relative to the preceding series value. Stacking a mix of negative and positive values will cause the areas to overlap. It is important to note that the interpolateNulls option does not work with stacked area charts.
Stepped area charts also support stacking, including 100% stacking. A stacked stepped area chart is a stepped area chart that places related values atop one another. If there are any negative values, they are stacked in reverse order below the chart's baseline. For 100% stacking, the stacks of elements at each domain-value are rescaled such that they add up to 100%. The options for this are isStacked: 'percent', which formats each value as a percentage of 100%, and isStacked: 'relative', which formats each value as a fraction of 1. There is also an isStacked: 'absolute' option, which is functionally equivalent to isStacked: true.