Use in the LineChart.Config passed to LineChart.viewCustom.

chartConfig : LineChart.Config Data Msg
chartConfig =
  { ...
  , area = Area.default
  , ...
  }

No color below your lines.

Color the area below your lines. The color is always the color of your line, but you can pass the opacity.

See example here.

Stacks your values and colors the area in the line color. The color is always the color of your line, but you can pass the opacity.

See example here.

Warning: Right now, this only works if all your lines have the same set of x values and don't have missing data! If not, the area will not stack properly. It will be fixed sometime though!