I’m trying to render the streamlit-altair chart from this example, so I implemented the following code:

import altair as alt
import streamlit as st

from view.frontend.path import BasePath

chart_spec = {
    "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
    "description": "A bar chart with highlighting on hover and selecting on click. (Inspired by Tableau's interaction style.)",
    "data": {
        "values": [
            {"a": "A", "b": 28}, {"a": "B", "b": 55}, {"a": "C", "b": 43},
            {"a": "D", "b": 91}, {"a": "E", "b": 81}, {"a": "F", "b": 53},
            {"a": "G", "b": 19}, {"a": "H", "b": 87}, {"a": "I", "b": 52},
        ]
    },
    "params": [
        {
            "name": "highlight",
            "select": {"type": "point", "on": "pointerover"}
        },
        {"name": "select", "select": "point"}
    ],
    "mark": {
        "type": "bar",
        "fill": "#4C78A8",
        "stroke": "black",
        "cursor": "pointer",
    },
    "encoding": {
        "x": {"field": "a", "type": "ordinal"},
        "y": {"field": "b", "type": "quantitative"},
        "fillOpacity": {
            "condition": {"param": "select", "value": 1},
            "value": 0.3,
        },
        "strokeWidth": {
            "condition": [
                {
                    "param": "select",
                    "empty": False,
                    "value": 2,
                },
                {
                    "param": "highlight",
                    "empty": False,
                    "value": 1,
                }
            ],
            "value": 0
        }
    },
    "config": {
        "scale": {"bandPaddingInner": 0.2}
    }
}

class Path(BasePath):
    def __init__(self, payload: dict):
        self._path_spec = payload

    def render(self, container):
        with container:
            # Altair Chart
            chart = alt.Chart().from_dict(chart_spec)

            # Display chart in Streamlit
            click_event = st.altair_chart(
                chart, use_container_width=True, on_select="rerun"
            )

            # Catch the click and display result
            if click_event is not None:
                st.write(f"Clicked on: {click_event}")

The problem is that I’m getting an empty chart in my app unlike the example page that is full with data.

What am I doing wrong?