Have a new function similar to alt.Title for adding a caption to a chart (alt.Caption)

[ChrisHassan1]

I would like to add captions to my charts which would appear at the bottom of a diagram. For the time being, I'm concatenating two charts together to add a caption

Example of my work-around for adding a caption at the bottom of a chart

frame='bounds' option: ensuring Title and Caption aligned:

df = pd.DataFrame(dict(x=list('abcde'), y=[1] * 5))
caption_df = pd.DataFrame(dict(caption=[['this is the start of the caption', 'this is end of the caption']]))


base_chart = alt.Chart(df).mark_point().encode(
    x=alt.X('x', title=None),
    y=alt.Y('y', title=None)
).properties(
    width=200, height=50
)

chart = base_chart.properties(
    title=alt.Title(
        text="Chart's Title",
        align='left',   # text=left to right aligned
        anchor='start', # text=starts at the beginning 
        subtitle=[
            "Chart's subtitle",
            "Using frame='bounds' option"
        ],    
        frame='group', #'group' | 'bounds',
    )
)

caption_chart = alt.Chart(
    caption_df
).mark_text(
    align='left',
    baseline='bottom',
    dx=-10 # trick to get it to align with y-axis
).encode(
    text='caption:N'
)


final_chart = alt.vconcat(
    chart,
    caption_chart,
    bounds='full', # 'full', 'flush'
    center=False, # still will not ensure that the caption text aligns with the title 
    spacing=10, # reduce spacing between chart and caption
)


final_chart

NOTE: the dx=-10 trick I am using to control where the text appears, which I repeat below

frame='bounds' option: ensuring Title and Caption aligned:

I re-use the dx trick but set it to dx=-30 this time

chart = base_chart.properties(
    title=alt.Title(
        text="Chart's Title",
        align='left',   # text=left to right aligned
        anchor='start', # text=starts at the beginning 
        subtitle=[
            "Chart's subtitle",
            "Using frame='bounds' option"
        ],    
        frame='bounds', #'group' | 'bounds',
    )
)

caption_chart = alt.Chart(
    caption_df
).mark_text(
    align='left',
    baseline='bottom',
    dx=-30 # trick to get it to align with y-axis
).encode(
    text='caption:N'
)


final_chart = alt.vconcat(
    chart,
    caption_chart,
    bounds='full', # 'full', 'flush'
    center=False, # still will not ensure that the caption text aligns with the title 
    spacing=10, # reduce spacing between chart and caption
)


final_chart

alt.Caption? As an alternative

It would have been great if a similar API was developed for adding a caption as with adding a title

i.e. if I could have done the below it would remove the need of concatenating two charts. I'm not sure what the best args would be for the caption - but mirroring what is used for the title. (assumption is that a caption always appears at the bottom, while title param always refers to text appearing above the chart)

chart = base_chart.properties(
    title=alt.Title(
        text="Chart's Title",
        align='left',   # text=left to right aligned
        anchor='start', # text=starts at the beginning 
        subtitle=[
            "Chart's subtitle",
            "Using frame='bounds' option"
        ],    
        frame='bounds', #'group' | 'bounds',
    ),
    caption=alt.Caption(
        text='this is the start of the caption',
        subtitle=['this is end of the caption'],
        align='left',
        anchor='start',
        frame='bounds',
    )
)