tweak(line): tweak #13013 & #13501.

- only make line bolder when there is more than one series and blur state
- or the user manually specify `lineStyle.width` as `bolder`

src/chart/line/LineSeries.ts

@@ -163,10 +163,7 @@ class LineSeriesModel extends SeriesModel<LineSeriesOption> {
         },
 
         emphasis: {
-            scale: true,
-            lineStyle: {
-                width: 'bolder'
-            }
+            scale: true
         },
         // areaStyle: {
             // origin of areaStyle. Valid values:

src/chart/line/LineView.ts

@@ -736,10 +736,7 @@ class LineView extends ChartView {
 
         setStatesStylesFromModel(polyline, seriesModel, 'lineStyle');
 
-        if (polyline.style.lineWidth > 0 && seriesModel.get(['emphasis', 'lineStyle', 'width']) === 'bolder') {
-            const emphasisLineStyle = polyline.getState('emphasis').style;
-            emphasisLineStyle.lineWidth = polyline.style.lineWidth + 1;
-        }
+        this._bolderLine(polyline, seriesModel, ecModel);
 
         // Needs seriesIndex for focus
         getECData(polyline).seriesIndex = seriesModel.seriesIndex;
@@ -1171,6 +1168,28 @@ class LineView extends ChartView {
         }
     }
 
+    /**
+     * To determine if the line should be bolder
+     *
+     * TODO:
+     * It's better to disable bolder when data is large,
+     * but it seems no simple way to figure out.
+     */
+    _bolderLine(polyline: ECPolyline, seriesModel: LineSeriesModel, ecModel: GlobalModel) {
+        if (polyline.style.lineWidth > 0) {
+            // only make line bolder when there is more than one series and blur state
+            // or the user manually specify `lineStyle.width` as `bolder`
+            // see https://github.com/apache/echarts/pull/13501
+            const emphasisLineWidth = seriesModel.get(['emphasis', 'lineStyle', 'width']);
+            if (emphasisLineWidth === 'bolder'
+                || (emphasisLineWidth == null && polyline.getState('blur') && ecModel.getSeriesCount() > 1)
+            ) {
+                const emphasisLineStyle = polyline.getState('emphasis').style;
+                emphasisLineStyle.lineWidth = polyline.style.lineWidth + 1;
+            }
+        }
+    }
+
     /**
      * @private
      */