Add RadialGrid charts

@@ -4,6 +4,7 @@
 
 - Fix some linting issues
 - Add PdfPage.rotate attribute
+- Add RadialGrid for charts with polar coordinates
 
 ## 3.0.1
 

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import 'dart:math' as math;
+
+import 'package:meta/meta.dart';
+import 'package:pdf/pdf.dart';
+
+import '../geometry.dart';
+import '../widget.dart';
+import 'chart.dart';
+
+@experimental
+class RadialGrid extends ChartGrid {
+  RadialGrid();
+
+  late PdfRect gridBox;
+
+  @override
+  void layout(Context context, BoxConstraints constraints,
+      {bool parentUsesSize = false}) {
+    super.layout(context, constraints, parentUsesSize: parentUsesSize);
+
+    final datasets = Chart.of(context).datasets;
+    final size = constraints.biggest;
+
+    gridBox = PdfRect(0, 0, size.x, size.y);
+
+    for (final dataset in datasets) {
+      dataset.layout(context, BoxConstraints.tight(gridBox.size));
+    }
+  }
+
+  @override
+  PdfPoint toChart(PdfPoint p) {
+    const z = 3.0;
+    return PdfPoint(
+      z * p.y * math.cos(p.x / 7 * math.pi * 2) + gridBox.width / 2,
+      z * p.y * math.sin(p.x / 7 * math.pi * 2) + gridBox.height / 2,
+    );
+  }
+
+  void paintBackground(Context context) {}
+
+  void clip(Context context, PdfPoint size) {
+    context.canvas
+      ..saveContext()
+      ..drawBox(gridBox)
+      ..clipPath();
+  }
+
+  @override
+  void paint(Context context) {
+    super.paint(context);
+
+    final datasets = Chart.of(context).datasets;
+
+    clip(context, box!.size);
+    for (var dataSet in datasets) {
+      dataSet.paintBackground(context);
+    }
+    context.canvas.restoreContext();
+    paintBackground(context);
+    clip(context, box!.size);
+    for (var dataSet in datasets) {
+      dataSet.paint(context);
+    }
+    context.canvas.restoreContext();
+  }
+}

@@ -25,6 +25,7 @@ export 'src/widgets/chart/bar_chart.dart';
 export 'src/widgets/chart/chart.dart';
 export 'src/widgets/chart/grid_axis.dart';
 export 'src/widgets/chart/grid_cartesian.dart';
+export 'src/widgets/chart/grid_radial.dart';
 export 'src/widgets/chart/legend.dart';
 export 'src/widgets/chart/line_chart.dart';
 export 'src/widgets/clip.dart';