Add mapToObject to handle nested data

[skip ci]

panel/models/vega.ts

@@ -10,6 +10,20 @@ import {set_size} from "./layout"
 import {debounce} from  "debounce"
 
 
+function mapToObject(map: Map<any, any>): any {
+  const obj: Record<string, any> = {};
+
+  map.forEach((value, key) => {
+    if (value instanceof Map) {
+      obj[key] = mapToObject(value);
+    } else {
+      obj[key] = value;
+    }
+  });
+
+  return obj;
+}
+
 export class VegaEvent extends ModelEvent {
   constructor(readonly data: any) {
     super()
@@ -147,8 +161,9 @@ export class VegaPlotView extends LayoutDOMView {
       this.model.data['datasets'] = datasets
     }
     const config: any = {actions: this.model.show_actions, theme: this.model.theme};
+    const embed_data = mapToObject(this.model.data);
 
-    (window as any).vegaEmbed(this.container, this.model.data, config).then((result: any) => {
+    (window as any).vegaEmbed(this.container, embed_data, config).then((result: any) => {
       this.vega_view = result.view
       this._resize = debounce(() => this.resize_view(result.view), 50)
       const callback = (name: string, value: any) => this._dispatch_event(name, value)