How to apply an opacity to a scatter plot flutter - flutter

Question

I am using charts_flutter to draw a scatter plot.
So far, this is the relevant code I wrote:
class _VennState extends State<VennDiagramWidget> {
List<charts.Series<VennCircle, int>> circlesList;
static List<charts.Series<VennCircle, int>> _createRandomCircles() {
final circles = [
VennCircle('red', 1, 50, 0.8, 'Venn1', 15, 35),
VennCircle('black', 2, 150, 0.5, 'Venn2', 18, 20),
VennCircle('green', 3, 200, 1, 'Venn3', 25, 25),
];
return [
new charts.Series(
id: 'circles',
data: circles,
domainFn: (VennCircle venn, _) => venn.x_value,
measureFn: (VennCircle venn, _) => venn.y_value,
colorFn: (VennCircle venn, _) {
if (venn.circleColor == 'red') {
return charts.MaterialPalette.red.shadeDefault;
} else if (venn.circleColor == 'black') {
return charts.MaterialPalette.black;
}
return charts.MaterialPalette.green.shadeDefault;
},
radiusPxFn: (VennCircle venn, _) => venn.circleSize,
domainUpperBoundFn: (VennCircle venn, _) => 50,
domainLowerBoundFn: (VennCircle venn, _) => 0,
measureUpperBoundFn: (VennCircle venn, _) => 50,
measureLowerBoundFn: (VennCircle venn, _) => 0,
),
];
}
scatterPlot() {
return new charts.ScatterPlotChart(
circlesList,
animate: true,
primaryMeasureAxis: new charts.NumericAxisSpec(
showAxisLine: false,
renderSpec: new charts.NoneRenderSpec(),
),
domainAxis: new charts.NumericAxisSpec(
showAxisLine: false,
renderSpec: new charts.NoneRenderSpec(),
),
);
}
I couldn't find a way to change the opacity of each circle. (each circle can have a different opacity)
I came across this Opacity widget but I wasn't able to apply it - probably because it should be used on a widget.
On that note, how can I define VennCircle as widget and not an object?
EDIT: I was able to do that simply by modifying the colorFn function:
colorFn: (VennCircle venn, _) {
if (venn.circleColor == 'red') {
return charts.Color.fromOther(
color: charts.ColorUtil.fromDartColor(
Color.fromARGB(100, 255, 0, 0)));
} else if (venn.circleColor == 'black') {
return charts.ColorUtil.fromDartColor(Color.fromARGB(95, 0, 0, 0));
}
return charts.ColorUtil.fromDartColor(Color.fromARGB(95, 0, 255, 0));
},