Strength accessor

.babelrc

@@ -1,3 +1,3 @@
 {
-  "presets": [ "es2015" ]
+  "presets": [ "env" ]
 }

lib/forceCluster.js

@@ -7,28 +7,28 @@ function cluster (centers) {
 
   let nodes,
     centerpoints = [],
-    strength = 0.1,
+    strength = () => 0.1,
+    strengths = [],
     centerInertia = 0.0;
 
   // coerce centers accessor into a function
   if (typeof centers !== 'function') centers = () => centers;
 
   function force (alpha) {
-    // scale + curve alpha value
-    alpha *= strength * alpha;
-
-    let c, x, y, l, r;
+    let a, c, x, y, l, r;
     nodes.forEach((d, i) => {
+      // scale + curve alpha value
+      a = alpha * strengths[i]
       c = centerpoints[i];
       if (!c || c === d) return;
 
       x = d.x - c.x,
       y = d.y - c.y,
       l = Math.sqrt(x * x + y * y),
-      r = d.radius + (c.radius || 0);
+      r = (d.radius || 0) + (c.radius || 0);
 
       if (l && l != r) {
-        l = (l - r) / l * alpha;
+        l = (l - r) / l * a;
         d.x -= x *= l;
         d.y -= y *= l;
         c.x += (1 - centerInertia) * x;
@@ -43,7 +43,11 @@ function cluster (centers) {
     // populate local `centerpoints` using `centers` accessor
     let i, n = nodes.length;
     centerpoints = new Array(n);
-    for (i = 0; i < n; i++) centerpoints[i] = centers(nodes[i], i, nodes);
+    strengths = new Array(n);
+    for (i = 0; i < n; i++) {
+      centerpoints[i] = centers(nodes[i], i, nodes);
+      strengths[i] = strength(nodes[i], i, nodes);
+    }
   }
 
   /**
@@ -77,7 +81,17 @@ function cluster (centers) {
    * Strength of attraction to the cluster center node/position.
    */
   force.strength = _ => {
-    return _ == null ? strength : (strength = +_, force);
+    // return existing value if no value passed
+    if (_ == null) return strength;
+
+    // coerce strength accessor into a function
+    strength = typeof _ === 'function' ? _ : (n, i) => _[i];
+
+    // reinitialize
+    initialize();
+
+    // allow chaining
+    return force;
   };
 
   /**