These pages outline the chart configuration options, and the methods and properties of Highcharts objects.
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Fill opacity for the area. When you set an explicit fillColor,
the fillOpacity is not applied. Instead, you should define the
opacity in the fillColor with an rgba color definition. The
fillOpacity setting, also the default setting, overrides the alpha
component of the color setting.
In styled mode, the fill opacity can be set with the
.highcharts-area class name.