add js-plotly-plot class to plotly element (fixes #2519)

nicegui/elements/plotly.py

@@ -36,6 +36,7 @@ def __init__(self, figure: Union[Dict, go.Figure]) -> None:
 
         self.figure = figure
         self.update()
+        self._classes.append('js-plotly-plot')
 
     def update_figure(self, figure: Union[Dict, go.Figure]):
         """Overrides figure instance of this Plotly chart and updates chart on client side."""