make default for 'splitBy' configurable

pdb-js/src/app/visualization-page/visualization-page.component.ts

@@ -46,9 +46,7 @@ export class VisualizationPageComponent implements OnInit {
   plotView: PlotViewComponent;
     
   enableGallery = false;
-  splitBy = new FormControl(null, [
-    Validators.required
-  ]);
+  splitBy = null;
 
   constructor(private plotService: PlotService) {
   }
@@ -69,6 +67,7 @@ export class VisualizationPageComponent implements OnInit {
       });
     
       that.groupBy = that.tagFields.filter(val => filterDefaults.groupBy.includes(val.name));
+      that.splitBy = that.tagFields.find(val => filterDefaults.splitBy == val.name);
     });
     this.yAxisScale = "LOG10";