alt.Repeat - create a grid of charts

Hi Jake,

In the docs you have a nice repeating example that I am trying to learn from. But suppose I have a simple dataFrame like this:

df=pd.DataFrame({'year': [1,2,3], 'school 1': [10,20,30], 'school 2': [15,6,25], 'school 3': [120,202,302], 'school 4': [152,6,225]})

and I would like a 2x2 grid where for each chart “year” is on the x axis and each charts show a line for a given school. So basically, one field is held constant. I can accomplish this by doing the following, but is there a better, more “Altair-ic” way to do this?

y1=alt.Chart().mark_line(point=True).encode(
    x='year',
    y='school 1',
)

y2=alt.Chart().mark_line(point=True).encode(
    x='year',
    y='school 2',
)

y3=alt.Chart().mark_line(point=True).encode(
    x='year',
    y='school 3',
)

y4=alt.Chart().mark_line(point=True).encode(
    x='year',
    y='school 4',
)

alt.vconcat((y1|y2), (y3|y4), data=df)

[Alcampopiano]

For anyone looking for an example until this functionality is in VL/Altair. There are various solutions here and my attempt is the following. Note that the DataFrame in this case has three columns and is in long form (see df.melt)

numcols=3 # specify the number of columns you want 
all_categories=df['Category_Column'].unique() # array of strings to use as your filters and titles

rows=alt.vconcat(data=df)
numrows=int(np.ceil(len(all_categories) / numcols))
pointer=0
for _ in range(numrows):

  row=all_categories[pointer:pointer+numcols]
  cols=alt.hconcat()

  for a_chart in row:

     # add your layers here
     # line chart
     line=alt.Chart().mark_line(point=True).encode(
        x='variable',
        y='value'
     ).transform_filter(datum.Category_Column == a_chart).properties(
        title=a_chart, height=200, width=200)

     # text labels
     text=alt.Chart().mark_text().encode(
        x='variable', 
        y='value'
     ).transform_filter(datum.Category_Column == a_chart)

     both = line + text
     cols |= both

  rows &= cols
  pointer += numcols

rows

[mattijn]

I haven’t seen this documented on SO or GitHub issues, but one can also use repeat instead of column or row within repeat:

import altair as alt
from vega_datasets import data

cars = data.cars.url

alt.Chart(cars, width=200, height=150).mark_bar().encode(
    x=alt.X(alt.repeat('repeat'), type='quantitative', bin=alt.Bin(maxbins=20)),
    y='count()'
).repeat(
    repeat=["Horsepower", "Miles_per_Gallon", "Acceleration", "Displacement"], 
    columns=2
)