Update Metric2D to remove useless string conversion.

modules/plugin_event/report_tools.py

@@ -609,7 +609,7 @@ class Metric2D(BaseReport):
         self.store = store = Store(data=[], fields=[], sorters=[])
 
         for col in df.columns:
-            di = {"name": str(col)}
+            di = {"name": col}
             di["type"] = ("string" if col == field_y else "number")
             store.fields.append(di)
 
@@ -637,7 +637,7 @@ class Metric2D(BaseReport):
             if col == field_y:
                 continue
 
-            name = str(col)
+            name = col
             grid.columns.append({"text": name,
                                  "dataIndex": name,
                                  "align": "right",