code: function getTooltipPosition([mouseX, mouseY], [parentChartWidth, parentChartHeight]) {
              let tooltipX, tooltipY;
  
              if (hasEnoughHorizontalRoom(parentChartWidth, mouseX)) {
                  tooltipX = mouseX + tooltipOffset.x;
              } else {
                  tooltipX = mouseX - chartWidth - tooltipOffset.x - margin.right;
              }
  
              if (hasEnoughVerticalRoom(parentChartHeight, mouseY)) {
                  tooltipY = mouseY + tooltipOffset.y;
              } else {
                  tooltipY = mouseY - chartHeight - tooltipOffset.y - margin.bottom;
              }
  
              return [tooltipX, tooltipY];
          }
docs: Calculates the desired position for the tooltip
  @param  {Number} mouseX             Current horizontal mouse position
  @param  {Number} mouseY             Current vertical mouse position
  @param  {Number} parentChartWidth   Parent's chart width
  @param  {Number} parentChartHeight  Parent's chart height
  @return {Number[]}                  X and Y position
  @private