echarts x轴名称太长

echarts x轴名称太长了，隐藏一部分，鼠标移到名称上，显示全称

 function extension(mychart, type) {
   let extension = document.getElementById("extension");
   mychart.on('mouseover', function (params) {
     if (params.componentType == type) {
       extension.innerHTML = params.value;
       let mousePos = mouseMove();
       let x = mousePos.x - 180;
       let y = mousePos.y - 50;
       extension.style.cssText = "top:" + y + "px;left:" + x +"px;display:inline;";
     }
   });

   mychart.on('mouseout', function (params) {
     if (params.componentType == type) {
       extension.style.cssText = "display:none";
     }
   });
 };

 function mousePosition(ev){
   if(ev.pageX || ev.pageY){
     return {x:ev.pageX, y:ev.pageY};
   }
   return {
     x:ev.clientX + document.body.scrollLeft - document.body.clientLeft,
     y:ev.clientY + document.body.scrollTop - document.body.clientTop
   };
 }
 function mouseMove(ev){
   ev = ev || window.event;
   return mousePosition(ev);
 }

 图表中
 xAxis: {
   type: 'category',
   data: [],
   triggerEvent: true,
   axisLabel: {
     formatter: function (value) {
       let res = value;
       if (res.length > 5) {
         res = res.substring(0, 4) + "..";
       }
       return res;
     }
   },
 },

 #extension {
   position: absolute;
   font-size: 14px;
   border-radius: 5px;
   color: #fff;
   background: #000;
   filter:alpha(opacity=70);
   -moz-opacity:0.7;
   -khtml-opacity: 0.7;
   opacity: 0.7;
   padding:5px;
   display: none;
 }