feat(charts): stack bar chart basic

utils/functionUtils.js

@@ -0,0 +1,26 @@
+function identity(x) {
+  return x;
+}
+
+export function rollup(values, reduce, ...keys) {
+  return nest(values, identity, reduce, keys);
+}
+
+function nest(values, map, reduce, keys) {
+  return (function regroup(values, i) {
+    if (i >= keys.length) return reduce(values);
+    const groups = new Map();
+    const keyof = keys[i++];
+    let index = -1;
+    for (const value of values) {
+      const key = keyof(value, ++index, values);
+      const group = groups.get(key);
+      if (group) group.push(value);
+      else groups.set(key, [value]);
+    }
+    for (const [key, values] of groups) {
+      groups.set(key, regroup(values, i));
+    }
+    return map(groups);
+  })(values, 0);
+}