Update alt.renderers.enable('altair_saver', fmts=['vega-lite', 'png'])

app.py

@@ -58,6 +58,8 @@ def process(command):
     # from PIL import Image
     import altair as alt
     from altair_saver import save
+    alt.renderers.enable('altair_saver', fmts=['vega-lite', 'png'])
+
     df_ = pd.DataFrame(data={'x': [1, 2], 'y': [3, 4], "cos": [0.1, 0.5]})
     chart_df = alt.Chart(df_).mark_circle(size=60).encode(
         x='x',