docs: overlap details (#390)

docs/en-us/options/global.md

@@ -221,3 +221,32 @@ func PieWithDispatchAction() *charts.Pie {
 }
 ```
 
+Additionally, when you have different types of series, you need the `Overlap` functions.
+```js
+series: [
+    {
+      name: 'Precipitation',
+      type: 'bar',
+      data: [
+        2.6, 5.9, 9.0, 26.4, 28.7, 70.7, 175.6, 182.2, 48.7, 18.8, 6.0, 2.3
+      ]
+    },
+    {
+      name: 'Temperature',
+      type: 'line',
+      data: [2.0, 2.2, 3.3, 4.5, 6.3, 10.2, 20.3, 23.4, 23.0, 16.5, 12.0, 6.2]
+    }
+  ]
+```
+
+In go-echarts:
+
+```go
+    line := charts.NewLine()
+    bar := charts.NewBar()
+    ...
+    bar.Overlap(line)
+```
+
+It will add the line's all series into bar's and the `Options` is only obtained from bar's. 
+