text: // d3.tip
  // Copyright (c) 2013 Justin Palmer
  // ES6 / D3 v4 Adaption Copyright (c) 2016 Constantin Gavrilete
  // Removal of ES6 for D3 v4 Adaption Copyright (c) 2016 David Gotz
  //
  // Tooltips for d3.js SVG visualizations
  
  d3.functor = function functor(v) {
      return typeof v === "function" ? v : function() {
          return v;
      };
  };
  
  d3.tip = function() {
  
      var direction = d3_tip_direction,
          offset    = d3_tip_offset,
          html      = d3_tip_html,
          node      = initNode(),
          svg       = null,
          point     = null,
          target    = null
  
      function tip(vis) {
          svg = getSVGNode(vis)
          point = svg.createSVGPoint()
          document.body.appendChild(node)
      }
  
      // Public - show the tooltip on the screen
      //
      // Returns a tip
      tip.show = function() {
          var args = Array.prototype.slice.call(arguments)
          if(args[args.length - 1] instanceof SVGElement) target = args.pop()
  
          var content = html.apply(this, args),
              poffset = offset.apply(this, args),
              dir     = direction.apply(this, args),
              nodel   = getNodeEl(),
              i       = directions.length,
              coords,
              scrollTop  = document.documentElement.scrollTop || document.body.scrollTop,
              scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft
  
          nodel.html(content)
              .style('position', 'absolute')
              .style('opacity', 1)
              .style('pointer-events', 'all')
  
          while(i--) nodel.classed(directions[i], false)
          coords = direction_callbacks[dir].apply(this)
          nodel.classed(dir, true)
              .style('top', (coords.top +  poffset[0]) + scrollTop + 'px')
              .style('left', (coords.left + poffset[1]) + scrollLeft + 'px')
  
          return tip
      }
  
      // Public - hide the tooltip
      //
      // Returns a tip
      tip.hide = function() {
          var nodel = getNodeEl()
          nodel
              .style('opacity', 0)
              .style('pointer-events', 'none')
          return tip
      }
  
      // Public: Proxy attr calls to the d3 tip container.  Sets or gets attribute value.
      //
      // n - name of the attribute
      // v - value of the attribute
      //
      // Returns tip or attribute value
      tip.attr = function(n, v) {
          if (arguments.length < 2 && typeof n === 'string') {
              return getNodeEl().attr(n)
          } else {
              var args =  Array.prototype.slice.call(arguments)
              d3.selection.prototype.attr.apply(getNodeEl(), args)
          }
  
          return tip
      }
  
      // Public: Proxy style calls to the d3 tip container.  Sets or gets a style value.
      //
      // n - name of the property
      // v - value of the property
      //
      // Returns tip or style property value
      tip.style = function(n, v) {
          // debugger;
          if (arguments.length < 2 && typeof n === 'string') {
              return getNodeEl().style(n)
          } else {
              var args = Array.prototype.slice.call(arguments);
              if (args.length === 1) {
                  var styles = args[0];
                  Object.keys(styles).forEach(function(key) {
                      return d3.selection.prototype.style.apply(getNodeEl(), [key, styles[key]]);
                  });
              }
          }
  
          return tip
      }
  
      // Public: Set or get the direction of the tooltip
      //
      // v - One of n(north), s(south), e(east), or w(west), nw(northwest),
      //     sw(southwest), ne(northeast) or se(southeast)
      //
      // Returns tip or direction
      tip.direction = function(v) {
          if (!arguments.length) return direction
          direction = v == null ? v : d3.functor(v)
  
          return tip
      }
  
      // Public: Sets or gets the offset of the tip
      //
      // v - Array of [x, y] offset
      //
      // Returns offset or
      tip.offset = function(v) {
          if (!arguments.length) return offset
          offset = v == null ? v : d3.functor(v)
  
          return tip
      }
  
      // Public: sets or gets the html value of the tooltip
      //
      // v - String value of the tip
      //
      // Returns html value or tip
      tip.html = function(v) {
          if (!arguments.length) return html
          html = v == null ? v : d3.functor(v)
  
          return tip
      }
  
      // Public: destroys the tooltip and removes it from the DOM
      //
      // Returns a tip
      tip.destroy = function() {
          if(node) {
              getNodeEl().remove();
              node = null;
          }
          return tip;
      }
  
      function d3_tip_direction() { return 'n' }
      function d3_tip_offset() { return [0, 0] }
      function d3_tip_html() { return ' ' }
  
      var direction_callbacks = {
          n:  direction_n,
          s:  direction_s,
          e:  direction_e,
          w:  direction_w,
          nw: direction_nw,
          ne: direction_ne,
          sw: direction_sw,
          se: direction_se
      };
  
      var directions = Object.keys(direction_callbacks);
  
      function direction_n() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.n.y - node.offsetHeight,
              left: bbox.n.x - node.offsetWidth / 2
          }
      }
  
      function direction_s() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.s.y,
              left: bbox.s.x - node.offsetWidth / 2
          }
      }
  
      function direction_e() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.e.y - node.offsetHeight / 2,
              left: bbox.e.x
          }
      }
  
      function direction_w() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.w.y - node.offsetHeight / 2,
              left: bbox.w.x - node.offsetWidth
          }
      }
  
      function direction_nw() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.nw.y - node.offsetHeight,
              left: bbox.nw.x - node.offsetWidth
          }
      }
  
      function direction_ne() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.ne.y - node.offsetHeight,
              left: bbox.ne.x
          }
      }
  
      function direction_sw() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.sw.y,
              left: bbox.sw.x - node.offsetWidth
          }
      }
  
      function direction_se() {
          var bbox = getScreenBBox()
          return {
              top:  bbox.se.y,
              left: bbox.e.x
          }
      }
  
      function initNode() {
          var node = d3.select(document.createElement('div'))
          node
              .style('position', 'absolute')
              .style('top', 0)
              .style('opacity', 0)
              .style('pointer-events', 'none')
              .style('box-sizing', 'border-box')
  
          return node.node()
      }
  
      function getSVGNode(el) {
          el = el.node()
          if(el.tagName.toLowerCase() === 'svg')
              return el
  
          return el.ownerSVGElement
      }
  
      function getNodeEl() {
          if(node === null) {
              node = initNode();
              // re-add node to DOM
              document.body.appendChild(node);
          };
          return d3.select(node);
      }
  
      // Private - gets the screen coordinates of a shape
      //
      // Given a shape on the screen, will return an SVGPoint for the directions
      // n(north), s(south), e(east), w(west), ne(northeast), se(southeast), nw(northwest),
      // sw(southwest).
      //
      //    +-+-+
      //    |   |
      //    +   +
      //    |   |
      //    +-+-+
      //
      // Returns an Object {n, s, e, w, nw, sw, ne, se}
      function getScreenBBox() {
          var targetel   = target || d3.event.target;
  
          while ('undefined' === typeof targetel.getScreenCTM && 'undefined' === targetel.parentNode) {
              targetel = targetel.parentNode;
          }
  
          var bbox       = {},
              matrix     = targetel.getScreenCTM(),
              tbbox      = targetel.getBBox(),
              width      = tbbox.width,
              height     = tbbox.height,
              x          = tbbox.x,
              y          = tbbox.y
  
          point.x = x
          point.y = y
          bbox.nw = point.matrixTransform(matrix)
          point.x += width
          bbox.ne = point.matrixTransform(matrix)
          point.y += height
          bbox.se = point.matrixTransform(matrix)
          point.x -= width
          bbox.sw = point.matrixTransform(matrix)
          point.y -= height / 2
          bbox.w  = point.matrixTransform(matrix)
          point.x += width
          bbox.e = point.matrixTransform(matrix)
          point.x -= width / 2
          point.y -= height / 2
          bbox.n = point.matrixTransform(matrix)
          point.y += height
          bbox.s = point.matrixTransform(matrix)
  
          return bbox
      }
  
      return tip
  };
  