Example gallery should show how to mark and label the final value in a timeseries

[palewire]

Charts like this have become commonplace in the COVID pandemic. It would be great to include an example showing how to do it. I'd submit a pull request, but I don't know how to do it! If someone can show me how, I'll submit an example for the docs.

[joelostblom]

Agree! I think direct labeling is often preferred over legends and I recently came across this example (bottom of the page) in the VegaLite docs that uses argmax for this, which is nicer than the more manual pandas approaches that I have used myself previously. Would you like to make an attempt at translating that to Altair code in a PR?

[palewire]

Good idea. How's this look?

import altair as alt
from vega_datasets import data

source = data.stocks()

chart = alt.Chart(source).transform_filter(
    alt.datum.symbol != "IBM"  # A reducation of the dataset to clarify our example. Not required.
).encode(
    color=alt.Color("symbol", legend=None)
)

line = chart.mark_line().encode(
    x="date:T",
    y="price:Q"
)

label = chart.encode(
    x=alt.X('date:T', aggregate='max'),
    y=alt.Y('price:Q', aggregate={'argmax': 'date'}),
    text='symbol'
)

text = label.mark_text(align='left', dx=4)

circle = label.mark_circle()

line + circle + text

[palewire]

The PR is submitted in #2623