Setting Custom Domain on datetime Axis

I’m trying to set the domain on my x-axis for a time based line/area chart. I’ve tried a few different things based on other issues raised here- but this is my closest result:

y = list(range(0,100))
x = [datetime.datetime(2017, 1, 1) + datetime.timedelta(days=i) for i in y]

# What I wanted to do
# domain = [datetime.datetime(2017,1,5), datetime.datetime(2017,1,30)]
# Per https://github.com/altair-viz/altair/issues/558
# domain_seconds = [(x - datetime.datetime(1970,1,1)).total_seconds() for x in domain]
# Per comment: https://github.com/altair-viz/altair/issues/187#issuecomment-244126070
domain_pd = pd.to_datetime(['2016-12-15', '2017-01-15']).astype(int) / 10 ** 6

time_data = pd.DataFrame({
    'x': x,
    'y': y
})

alt.Chart(time_data).mark_line().encode(
    alt.X('x:T', timeUnit='yearmonthdate', scale=alt.Scale(domain=list(domain_pd))),
    alt.Y('y:Q')
).properties(
    title='Example chart'
)

As you can see, this didn’t exactly work out. What I really want to do is pass in dates in the domain field and have the chart start and end on those dates. This should happen regardless of the presence of data. For my application there may or may not be data in the dates given. Am I passing in these dates in the right place?

FYI I’m running altair 2.1.0 in a jupyter notebook.

I’m tracking the following issues to try and resolve this:

• https://github.com/altair-viz/altair/issues/558
• https://github.com/altair-viz/altair/issues/187#issuecomment-244126070

[jakevdp]

Just thinking out loud… sorry for the multiple posts.

Maybe the best way to do this would be to make alt.DateTime accept a flexible range of inputs, so that the user could do

alt.Scale(domain=[alt.DateTime('2016-12-15'), alt.DateTime('2017-01-15')])

That seems concise and convenient, and also nice and explicit.

Edit: This would involve adding the following class to altair/vegalite/v2/api/py:

import altair as alt
import pandas as pd

class DateTime(alt.DateTime):
    @staticmethod
    def _shorthand_to_dict(shorthand):
        dt = pd.to_datetime(shorthand)
        return dict(year=dt.year, month=dt.month, date=dt.day,
                    hours=dt.hour, minutes=dt.minute, seconds=dt.second,
                    milliseconds=0.001 * dt.microsecond)
    
    def __init__(self, shorthand=alt.Undefined, **kwargs):
        if shorthand is not alt.Undefined:
            # TODO: warn if this involves overwriting
            kwargs.update(self._shorthand_to_dict(shorthand))
        super(alt.DateTime, self).__init__(**kwargs)

[SyntaxRules]

Nice to see vega-lite will be making this a little easier. If I understand right after vega-lite incorporates this into a release (is this in 2.6.0?) altair can handle:

domain=['2016-12-15', '2017-01-15']
# or
domain=[datetime.date(2016, 12, 15).isoformat(), datetime.date(2017, 1, 15).isoformat()]

I’m looking forward to trying this out.