fix g2line performance issue

src/components/G2Plot/G2Line.vue

 <template>
-  <div :ref="id" style="height: 100%;width: 100%" />
+  <div :id="id" style="height: 100%;width: 100%" />
 </template>
 
 <script>
@@ -32,13 +32,25 @@ export default {
     }
   },
   watch: {
+    title: function(val) {
+      this.line.updateConfig({
+        title: {
+          visible: true,
+          text: val
+        },
+        description: {
+          visible: true,
+          text: `单位为: ${this.description}`
+        }
+      })
+      this.line.render()
+    },
     lineData: function(val) {
       this.line.changeData(val)
     }
   },
   mounted() {
-    const el = this.$refs[this.id]
-    this.line = new Line(el, {
+    this.line = new Line(document.getElementById(this.id), {
       data: this.lineData,
       title: {
         visible: true,
@@ -57,7 +69,13 @@ export default {
         crosshairs: {
           type: 'y'
         },
-        offset: 20
+        offset: 20,
+        formatter: (x, y) => {
+          return {
+            name: this.title,
+            value: `${y} ${this.description}`
+          }
+        }
       },
       description: {
         visible: true,
@@ -74,6 +92,9 @@ export default {
       ]
     })
     this.line.render()
+  },
+  beforeDestroy() {
+    this.line.destroy()
   }
 }
 </script>