How to solve strange plotting error in Altair

Question

I'm trying to plot some data which broadly should form a map like so:

import numpy as np
lats = np.random.uniform(51.5, 51.6, 100)
lons = np.random.uniform(-0.1, 0.1, 100)
months = np.arange(1, 13)
vouchers = np.random.randint(1, 100, 100)
test_df = pd.DataFrame({
    'lat': lats,
    'lon': lons,
    'month': np.random.choice(months, 100),
    'vouchers': vouchers
})
test_df 

alt.Chart(test_df).mark_circle().encode(
    longitude='lon:Q',
    latitude='lat:Q',
    size='vouchers:Q',
    color='month:N'
)

That looks like this:

Anyway, my real data is very similar.

Describe shows this:

and the data is the right dtypes:

Here's my code:

alt.Chart(lunch_vouchers).mark_circle().encode(
    longitude='long_jittered:Q',
    latitude='lat_jittered:Q',
    size=alt.Size('Total_People_in_voucher:Q', scale=alt.Scale(range=[0, 1000]), legend=None),
    color=alt.value('red'),
    tooltip=['Postcode', 'Total_People_in_voucher', 'Month']
)

But it produces a strange display error in VS Code.

How can I fix this?