Altair plot not showing when using Panel

I am following this tutorial [github] on using Panel and Altair.

The function which returns my plot is the following:

@pn.depends(ticker.param.value, date_range_slider.param.value)
def get_plot(ticker, date_range):

    df = data.stocks()
    df['date'] = pd.to_datetime(df['date'])

    start_date, end_date = date_range

    mask = (df['date'] > start_date) & (df['date'] <= end_date)
    dfm = df.loc[mask]

    chart = (alt.Chart(dfm)
        .mark_line()
        .encode(
            x='date', y='price',
            tooltip=alt.Tooltip(['date', 'price']))
        .transform_filter((datum.symbol == ticker))
    )
    return chart

I am running into an issue where the chart is shown normally if executed directly:

but not when run using Panel:

It seems that when i put my working chart into a panel it loses the link to the data:

Serving the dashboard has the same problem so it seems to me there is a compatibility issue between Panel and Altair.

Anyone know what is wrong and how to fix it?

Versions:

• Panel - v0.6.0
• Altair - v3.2.0
• Pandas - v0.25.1
• JupyterLab - v1.0.2
• Python - v3.7.4

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