How can I exclude certain dates (e.g., weekends) from time series plots?

Question

In the following example, I'd like to exclude weekends and plot Y as a straight line, and specify some custom frequency for major tick labels since they would be a "broken" time series (e.g., every Monday, a la matplotlib's set_major_locator).

How would I do that in Altair?

import altair as alt
import pandas as pd

index = pd.date_range('2018-01-01', '2018-01-31', freq='B')
df = pd.DataFrame(pd.np.arange(len(index)), index=index, columns=['Y'])

alt.Chart(df.reset_index()).mark_line().encode(
    x='index',
    y='Y'
)

Answer 1

A quick way to do that is to specify the axis as an ordinal field. This would produce a very ugly axis, with the hours specified for every tick. To change that, I add a column to the dataframe with a given label. I also added the grid, as by default it is removed for an ordinal encoding, and set the labelAngle to 0.

df2 = df.assign(label=index.strftime('%b %d %y'))

alt.Chart(df2).mark_line().encode(
    x=alt.X('label:O', axis=alt.Axis(grid=True, labelAngle=0)),
    y='Y:Q'
)

Beware that it would remove any missing point. So, maybe you want to add a tooltip. This is discussed in the documentation here. You can also play with labelOverlap in the axis setting depending of hat you want.

---

To customize the axis, we can build one up using mark_text and bring back the grid with mark_rule and a custom dataframe. It does not necessarily scale up well, but it can give you some ideas.

df3 = df2.loc[df2.index.dayofweek == 0, :].copy()
df3["Y"] = 0

text_chart = alt.Chart(df3).mark_text(dy = 15).encode(
    x=alt.X('label:O', axis = None),
    y=alt.Y('Y:Q'),
    text=alt.Text('label:O')
)

tick_chart = alt.Chart(df3).mark_rule(color='grey').encode(
    x=alt.X('label:O', axis=None),
)

line_chart = alt.Chart(df2).mark_line().encode(
    x=alt.X('label:O', axis=None, scale=alt.Scale(rangeStep=15)),
    y='Y:Q'
)
text_chart + tick_chart + line_chart