Add flutter_web samples (#75)

web/github_dataviz/lib/data/week_label.dart

@@ -0,0 +1,24 @@
+import 'package:intl/intl.dart';
+
+class WeekLabel {
+  int weekNum;
+  String label;
+
+  WeekLabel(this.weekNum, this.label);
+
+  WeekLabel.forDate(DateTime date, String label) {
+    this.label = label;
+    int year = getYear(date);
+    int weekOfYearNum = getWeekNumber(date);
+    this.weekNum = 9 + ((year - 2015) * 52) + weekOfYearNum;
+  }
+
+  int getYear(DateTime date) {
+    return int.parse(DateFormat("y").format(date));
+  }
+
+  int getWeekNumber(DateTime date) {
+    int dayOfYear = int.parse(DateFormat("D").format(date));
+    return ((dayOfYear - date.weekday + 10) / 7).floor();
+  }
+}