custom sorting categorical axis with layers - working in VL3 Editor

Hi Jake,

Not sure if you’ve seen the recent slack discussion on this but I wanted to let you know that the following works in the VL3 Editor but not in Altair. I’m referring to the sorting of the Y axis labels.

df=pd.DataFrame({'Org': ['school','school','school','school', 'board', 'board', 'board', 'board'],
                 'variable': ['rose', 'dropped', 'maintained', 'failed','rose', 'dropped', 'maintained', 'failed'],
                'percent': [25,25,25,25,25,25,25,25], 
                 'position': [12.5, 37.5,62.5, 87.5,12.5, 37.5,62.5, 87.5]})
    
bars=alt.Chart().mark_bar().encode(
    x=alt.X('sum(percent)', title='Percentage of Students'),
    y=alt.Y('Org', sort=['school'], title=None),
    color=alt.Color('variable', title=None),
    order='junk:Q'
)

text=alt.Chart().mark_text(align='center', baseline='middle').encode(
    y=alt.Y('Org', sort=['school'], title=None),
    x='position',
    text='percent')

alt.layer(bars, text, data=df).transform_calculate(
    junk="if(datum.variable === 'maintained', 0, if(datum.variable === 'rose', 1, if(datum.variable === 'dropped', 2, 2)))")

The VL folks showed me how to get this working and I thought perhaps it would translate to Altair 2.2. I assume it is simply a VL2 versus VL3 difference but wanted to bring it to your attention in case it is helpful for you to know.

[jtbaker]

As of v2.3.0 with VL 3, axes may now be sorted by passing a custom sort array to the channel as such:

chart = alt.Chart(df, height=210).mark_rect().encode(
    x=alt.X("hour:N", title='Hour'), 
    y=alt.Y("dayofweek:O", sort=["Sun", "Mon", "Tue", "Wed", "Thurs", "Fri", "Sat"],
            title="Day of Week"),
    color=alt.Color(
        "cnt:Q", 
        scale=alt.Scale( range=['lightyellow','red']), 
        legend=alt.Legend(title='Count')
    ),
    tooltip=[
        alt.Tooltip("cnt:Q", title="Count")
    ]
)

[afonit]

In the ‘bars’ chart, for the y put this:

y=alt.Y('Org', sort=alt.EncodingSortField(field='Org', op='sum', order='descending'), title=None),

That now sorts ‘board’ before ‘school’ on the y-axis.