Remove force.gravityCenter.

Simpler to perform gravity calculations in a custom tick handler.

d3.layout.js

@@ -215,7 +215,6 @@ d3.layout.force = function() {
       linkStrength = d3_layout_forceLinkStrength,
       charge = -30,
       gravity = .1,
-      gravityCenter = null,
       theta = .8,
       interval,
       nodes = [],
@@ -283,11 +282,6 @@ d3.layout.force = function() {
     y = size[1] / 2;
     i = -1; while (++i < n) {
       o = nodes[i];
-      if (gravityCenter != null) {
-        var gxy = gravityCenter.call(this, o, i);
-        x = gxy.x;
-        y = gxy.y;
-      }
       o.x += (x - o.x) * kg;
       o.y += (y - o.y) * kg;
     }
@@ -375,12 +369,6 @@ d3.layout.force = function() {
     return force;
   };
 
-  force.gravityCenter = function(x) {
-    if (!arguments.length) return gravityCenter;
-    gravityCenter = x == null ? null : d3.functor(x);
-    return force;
-  };
-
   force.theta = function(x) {
     if (!arguments.length) return theta;
     theta = x;

examples/cartogram/demers.js

@@ -13,9 +13,6 @@ var color = d3.scale.linear()
 var force = d3.layout.force()
     .charge(0)
     .gravity(0)
-    .gravityCenter(function(node) {
-      return node.gravity;
-    })
     .size([960, 500]);
 
 var svg = d3.select("#chart").append("svg:svg");
@@ -30,7 +27,7 @@ d3.json("../data/us-state-centroids.json", function(states) {
       x: xy[0],
       y: xy[1],
       gravity: {x: xy[0], y: xy[1]},
-      r: Math.sqrt(data[+d.id] * 5000),
+      r: Math.sqrt(data[+d.id] * 2500),
       value: data[+d.id]
     };
   });
@@ -40,9 +37,13 @@ d3.json("../data/us-state-centroids.json", function(states) {
       .links(links)
       .start()
       .on("tick", function(e) {
-    var k = e.alpha;
+    var k = e.alpha * .5,
+        kg = k * .1;
     nodes.forEach(function(a, i) {
-      nodes.slice(i + 1).forEach(function(b, i) {
+      // Apply gravity forces.
+      a.x += (a.gravity.x - a.x) * kg;
+      a.y += (a.gravity.y - a.y) * kg;
+      nodes.slice(i + 1).forEach(function(b) {
         // Check for collision
         var dx = a.x - b.x,
             dy = a.y - b.y,

examples/cartogram/dorling.js

@@ -13,9 +13,6 @@ var color = d3.scale.linear()
 var force = d3.layout.force()
     .charge(0)
     .gravity(0)
-    .gravityCenter(function(node) {
-      return node.gravity;
-    })
     .size([960, 500]);
 
 var svg = d3.select("#chart").append("svg:svg")
@@ -44,9 +41,13 @@ d3.json("../data/us-state-centroids.json", function(states) {
       .links(links)
       .start()
       .on("tick", function(e) {
-    var k = e.alpha;
+    var k = e.alpha,
+        kg = k * .02;
     nodes.forEach(function(a, i) {
-      nodes.slice(i + 1).forEach(function(b, i) {
+      // Apply gravity forces.
+      a.x += (a.gravity.x - a.x) * kg;
+      a.y += (a.gravity.y - a.y) * kg;
+      nodes.slice(i + 1).forEach(function(b) {
         // Check for collisions.
         var dx = a.x - b.x,
             dy = a.y - b.y,

src/layout/force.js

@@ -9,7 +9,6 @@ d3.layout.force = function() {
       linkStrength = d3_layout_forceLinkStrength,
       charge = -30,
       gravity = .1,
-      gravityCenter = null,
       theta = .8,
       interval,
       nodes = [],
@@ -77,11 +76,6 @@ d3.layout.force = function() {
     y = size[1] / 2;
     i = -1; while (++i < n) {
       o = nodes[i];
-      if (gravityCenter != null) {
-        var gxy = gravityCenter.call(this, o, i);
-        x = gxy.x;
-        y = gxy.y;
-      }
       o.x += (x - o.x) * kg;
       o.y += (y - o.y) * kg;
     }
@@ -169,12 +163,6 @@ d3.layout.force = function() {
     return force;
   };
 
-  force.gravityCenter = function(x) {
-    if (!arguments.length) return gravityCenter;
-    gravityCenter = x == null ? null : d3.functor(x);
-    return force;
-  };
-
   force.theta = function(x) {
     if (!arguments.length) return theta;
     theta = x;