title: How to plot multiple Y axis using Altair on Streamlit?
body: <p>I am wondering how I can plot multiple Y axis using Altair on Streamlit.</p>
  <pre><code>import pandas as pd
  import numpy as np
  import altair as alt
  import streamlit as st
  
  df = pd.DataFrame({
      'name': ['brian', 'dominik', 'patricia'],
      'age': [20, 30, 40],
      'salary': [1000, 2000, 3000]
  })
  
  c = alt.Chart(df).mark_area().encode(
      x='name', y=['age', 'salary'])
  st.altair_chart(c, use_container_width=True)
  </code></pre>
  <p>The above example returns the following error:</p>
  <p>SchemaValidationError: Invalid specification altair.vegalite.v4.schema.core.FacetedEncoding-&gt;0, validating 'type' [{'type': 'quantitative', 'field': 'age'}, {'type': 'quantitative', 'field': 'salary'}] is not of type 'object'</p>