Adding labels at end of line chart in Altair

Question

So I have been trying to get it so there is a label at the end of each line giving the name of the country, then I can remove the legend. Have tried playing with transform_filter but no luck.

I used data from here https://ourworldindata.org/coronavirus-source-data I cleaned and reshaped the data so it looks like this:-

    index   days    date    country value
0   1219    0   2020-03-26  Australia   11.0
1   1220    1   2020-03-27  Australia   13.0
2   1221    2   2020-03-28  Australia   13.0
3   1222    3   2020-03-29  Australia   14.0
4   1223    4   2020-03-30  Australia   16.0
5   1224    5   2020-03-31  Australia   19.0
6   1225    6   2020-04-01  Australia   20.0
7   1226    7   2020-04-02  Australia   21.0
8   1227    8   2020-04-03  Australia   23.0
9   1228    9   2020-04-04  Australia   30.0

import altair as alt

countries_list = ['Australia', 'China', 'France', 'Germany', 'Iran', 'Italy','Japan', 'South Korea', 'Spain', 'United Kingdom', 'United States']

chart = alt.Chart(data_core_sub).mark_line().encode(
                                            alt.X('days:Q'),
                                            alt.Y('value:Q', scale=alt.Scale(type='log')),
                                            alt.Color('country:N', scale=alt.Scale(domain=countries_list,type='ordinal')),    
                                        )

labels = alt.Chart(data_core_sub).mark_text().encode(
                                            alt.X('days:Q'),
                                            alt.Y('value:Q', scale=alt.Scale(type='log')),
                                            alt.Text('country'),
                                            alt.Color('country:N', legend=None, scale=alt.Scale(domain=countries_list,type='ordinal')), 
                                        ).properties(title='COVID-19 total deaths', width=600)
                                        

alt.layer(chart, labels).resolve_scale(color='independent')

This is the current mess that the chart is in.

How would I go about just showing the last 'country' name?

EDIT

Here is the result. I might look at adjusting some of the countries separately as adjusting as a group means that some of the labels are always badly positioned no matter what I do with the dx and dy alignment.

Answer 1

You can do this by aggregating the x and y encodings. You want the text to be at the maximum x value, so you can use a 'max' aggregate in x. For the y-value, you want the y value associated with the max x-value, so you can use an {"argmax": "x"} aggregate.

With a bit of adjustment of text alignment, the result looks like this:

labels = alt.Chart(data_core_sub).mark_text(align='left', dx=3).encode(
    alt.X('days:Q', aggregate='max'),
    alt.Y('value:Q', aggregate={'argmax': 'days'}, scale=alt.Scale(type='log')),
    alt.Text('country'),
    alt.Color('country:N', legend=None, scale=alt.Scale(domain=countries_list,type='ordinal')), 
).properties(title='COVID-19 total deaths', width=600)