'stroke', '#000')
        .attr('id', 'target2');

      d3target2_radius = svg.append('circle')
        .attr('cx', 300)
        .attr('cy', 300)
        .attr('r', 10)
        .attr('fill', 'rgba(255,0,0,0.1)')
        .attr('stroke', '#000');

      // draw line indicating forces
      d3line = svg.append('line')
        .attr('x1', 0)
        .attr('y1', 0)
        .attr('x2', 0)
        .attr('y2', 0)
        .attr('stroke', 'black')
        .attr('stroke-width', '2')
        .attr("marker-end", "url(#