Fixed a bug with switching viz_type in exploreV2 (#1631)

- Issue: when switching from a viz_type outside nvd3 to a viz_type in
 nvd3, the Chart Container doesn't draw new graph
 - Fix: The reason was somehow the function inside nv.addGraph() wasn't
   called, extract the function outside and explicitly calling it solve
   the problem

superset/assets/visualizations/nvd3_vis.js

@@ -104,7 +104,8 @@ function nvd3Vis(slice) {
       const reduceXTicks = fd.reduce_x_ticks || false;
       let stacked = false;
       let row;
-      nv.addGraph(function () {
+
+      const drawGraph = function () {
         switch (vizType) {
           case 'line':
             if (fd.show_brush) {
@@ -352,8 +353,10 @@ function nvd3Vis(slice) {
         }
 
         return chart;
-      });
+      };
 
+      const graph = drawGraph();
+      nv.addGraph(graph);
       slice.done(payload);
     });
   };