D3 force layout with CSS positioning - svg

Question

I'm placing circles on a map corresponding to GPS coordinates. Each circle is contained within an svg container which is placed on the page using CSS top and left properties. In my implementation, these containers often sit atop one another.
I am trying to implement collision detection and/or add a slight negative charge to these containers so that overlaps cause containers to distance themselves from one another.
Thus far, my tests with force layouts have either resulted in no change, or resulted in an error ('cannot set property index of null' or 'cannot set property x of null'). It's apparent that I'm doing something wrong but I have been unable to identify a path to resolution from the articles I've read online.
Any ideas on how I can stop the containers from sitting atop one another?
var self = this;
var data = [{lat: 127, lon: 36, name: 'a', radius: 9},{lat:127, lon: 36, name: 'b', radius: 9}];
// Position SVG containers correctly
var latLngToPx = function(d) {
var temp = new google.maps.LatLng(d.lat, d.lon);
temp = self.map.projection.fromLatLngToDivPixel(temp);
d.x = temp.x;
d.y = temp.y;
return d3.select(this)
.style('left', d.x + 'px')
.style('top', d.y + 'px');
};
var collide = function(node) {
var r = node.radius + 16,
nx1 = node.x - r,
nx2 = node.x + r,
ny1 = node.y - r,
ny2 = node.y + r;
return function(quad, x1, y1, x2, y2) {
if (quad.point && (quad.point !== node)) {
var x = node.x - quad.point.x,
y = node.y - quad.point.y,
l = Math.sqrt(x * x + y * y),
r = node.radius + quad.point.radius;
if (l < r) {
l = (l - r) / l * 0.5;
node.x -= x *= l;
node.y -= y *= l;
quad.point.x += x;
quad.point.y += y;
}
}
return x1 > nx2 || x2 < nx1 || y1 > ny2 || y2 < ny1;
};
}
var svgBind = d3.select(settings[type].layer).selectAll('svg')
.data(data, function(d){ return d.name; })
.each(latLngToPx);
var svg = svgBind.enter().append('svg')
.each(latLngToPx)
// svg[0] contains the svg elements
var nodes = svg[0];
var force = d3.layout.force()
.nodes(nodes)
.charge(-100)
.start();
force.on('tick', function(){
var q = d3.geom.quadtree(nodes),
i = 0,
n = nodes.length;
while (++i < n) {
q.visit(collide(nodes[i]));
}
svg
.style('left', function(d){ return (d.x - lm.config.offset) + 'px';})
.style('top', function(d){ return (d.y - lm.config.offset) + 'px';});
});
var circ = svg.append('circle')
.attr('r', settings[type].r)
.attr('cx',10)
.attr('cy',10)

Answer 1

You shouldn't need to do the collision detection yourself -- the force layout should take care of that for you. Here are the basic steps you need to take.
To each data element that represents a circle, add x and y members that contain their current (screen) coordinates. This is what the force layout will operate on.
Pass the array of these elements to the force layout as nodes. There's no need to set links to start with, although you might want to do so later to control the placement of nodes with respect to each other.
Start the force layout.
For each tick, redraw the elements at the appropriate position.
Tweak the parameters of the force layout to your liking.
You are doing most of this already, I'm just mentioning it again to clarify. The code would look something like this.
function latLngToPx(d) {
var temp = new google.maps.LatLng(d.lat, d.lon);
temp = self.map.projection.fromLatLngToDivPixel(temp);
d.x = temp.x;
d.y = temp.y;
};
data.forEach(function(d) { latLngToPx(d); });
var nodes = d3.select("body").selectAll("svg").data(data).enter().append("svg");
var force = d3.layout.force().nodes(data);
force.on("tick", function() {
nodes.style('left', function(d){ return (d.x - lm.config.offset) + 'px';})
.style('top', function(d){ return (d.y - lm.config.offset) + 'px';});
});