code: function centerGraphic(rect, boundingRect) {
      // Set rect to center, keep width / height ratio.
      var aspect = boundingRect.width / boundingRect.height;
      var width = rect.height * aspect;
      var height;
      if (width <= rect.width) {
          height = rect.height;
      }
      else {
          width = rect.width;
          height = width / aspect;
      }
      var cx = rect.x + rect.width / 2;
      var cy = rect.y + rect.height / 2;
  
      return {
          x: cx - width / 2,
          y: cy - height / 2,
          width: width,
          height: height
      };
  }
docs: Get position of centered element in bounding box.
  
  @param  {Object} rect         element local bounding box
  @param  {Object} boundingRect constraint bounding box
  @return {Object} element position containing x, y, width, and height