  .force('collision', d3.forceCollide().radius(3))
                .force('link', d3.forceLink().links(graph.links).distance(0))
                .force("charge", d3.forceManyBody().strength(-30))
                .force("x", d3.forceX())
                .force("y", d3.forceY())
                         .on('tick', ticked);

        } else if (btn == 'city') {

                        simulation
                .force('center', d3.forceCenter(width / 2, height / 2))
                .force('collision', d3.forc