code: function checkConnection_(yIndex) {
      var dx = currentX - db[yIndex].x_;
      var dy = currentY - db[yIndex].y_;
      var r = Math.sqrt(dx * dx + dy * dy);
      if (r <= maxRadius) {
        neighbours.push(db[yIndex]);
      }
      return dy < maxRadius;
    }
docs: Computes if the current connection is within the allowed radius of another
  connection.
  This function is a closure and has access to outside variables.
  @param {number} yIndex The other connection's index in the database.
  @return {boolean} True if the current connection's vertical distance from
  the other connection is less than the allowed radius.