Add flutter_web samples (#75)

web/github_dataviz/lib/mathutils.dart

@@ -0,0 +1,48 @@
+abstract class Interpolator {
+  double get(double x);
+}
+
+class EarlyInterpolator implements Interpolator {
+  double amount;
+
+  EarlyInterpolator(this.amount);
+
+  @override
+  double get(double x) {
+    if (x >= amount) {
+      return 1;
+    } else {
+      return MathUtils.map(x, 0, amount, 0, 1);
+    }
+  }
+}
+
+class Point2D {
+  double x, y;
+
+  Point2D(this.x, this.y);
+}
+
+class MathUtils {
+  static double map(double x, double a, double b, double u, double v) {
+    double p = (x - a) / (b - a);
+    return u + p * (v - u);
+  }
+
+  static double clampedMap(double x, double a, double b, double u, double v) {
+    if (x <= a) {
+      return u;
+    } else if (x >= b) {
+      return v;
+    } else {
+      double p = (x - a) / (b - a);
+      return u + p * (v - u);
+    }
+  }
+
+  static double clamp(double x, double a, double b) {
+    if (x < a) return a;
+    if (x > b) return b;
+    return x;
+  }
+}