How the radar chart is drawn in CustomPainter.

flutter_radar_chart_drawing.dart

void paint(ui.Canvas canvas, ui.Size size) {
    final Offset centerPoint = Offset(0, 0);

    // math for each branch
    final angle = 360 / nodes;
    // list of branch end points (x,y)
    List<List<double>> points = [];
    // for each branch
    for (int i = 0; i < nodes; i++) {
      final double lineLength = 200;
      // paint styles
      // branch
      final lineStyle = Paint()
        ..strokeWidth = 3
        ..color = Colors.black;

      // node
      final nodeRadius = 5;
      final nodeStyle = Paint()..color = Colors.black;

      if (i == 0) {
        // start at 12 o clock
        points.add([0, -lineLength]);
      } else {
        // everything else
        final currentAngle = i * angle * pi / 180;
        final x = lineLength * sin(currentAngle); // x
        final y = -lineLength * cos(currentAngle); // y
        points.add([x, y]);
      }
      // draw branch
      Offset endPoint = Offset(points[i][0], points[i][1]);
      canvas.drawLine(centerPoint, endPoint, lineStyle);
      // draw node
      canvas.drawCircle(endPoint, nodeRadius, nodeStyle);
    }

    // draw segments
    int totalSegments = segments * nodes;
    int i = 0;
    for (var j = 0; j < totalSegments; j++) {
      if (j >= 0 && j < nodes) {
        i++;
        if (i >= nodes) {
          i = 0;
        }
        continue; // skip nodes at center
      }
      // segment
      final segRadius = 2.5;
      final segStyle = Paint()..color = Colors.black;
      var segNo = (j / nodes).floor() / segments;
      Offset segPoint = Offset(points[i][0], points[i][1]) * segNo.toDouble();
      canvas.drawCircle(segPoint, segRadius, segStyle);
      i++;
      if (i >= nodes) {
        i = 0;
      }
    }

    // draw poly
    final polyStyle = Paint()
      ..strokeWidth = 2
      ..color = Colors.grey.withAlpha(100);
    List<Offset> dataOffsets = [];
    for (var i = 0; i < nodes; i++) {
      var temp = Offset(points[i][0], points[i][1]) * data[i];
      dataOffsets.add(temp);
    }
    Path polyPath = Path();
    polyPath.addPolygon(dataOffsets, true);
    canvas.drawPath(polyPath, polyStyle);

    // draw labels
    double fontHeight = 12.0;
    TextStyle style = TextStyle(
      color: Colors.grey.withAlpha((255).toInt()),
      fontSize: fontHeight,
      fontStyle: FontStyle.normal,
      fontWeight: FontWeight.normal,
      fontFamily: 'Open Sans',
    );

    for (var i = 0; i < nodes; i++) {
      final paraBuilder = ui.ParagraphBuilder(ui.ParagraphStyle(
        fontSize: style.fontSize,
        fontFamily: style.fontFamily,
        fontStyle: style.fontStyle,
        fontWeight: style.fontWeight,
        textAlign: TextAlign.center,
      ))
        ..pushStyle(style.getTextStyle());
      paraBuilder.addText(labels[i]);
      final ui.Paragraph labelPara = paraBuilder.build()
        ..layout(ui.ParagraphConstraints(width: size.width));
      var temp =
          Offset(points[i][0] - fontHeight / 4, points[i][1] - fontHeight / 2) *
              1.1;
      canvas.drawParagraph(labelPara, temp);
    }
  }