Add timeUnit support

mpl-altair currently doesn't support conversion for Altair charts that use timeUnit aggregations.

Code for reproduction

This and other timeUnit transformations don't work:

from vega_datasets import data
seattle = data.seattle_weather()

chart = alt.Chart(seattle).mark_point().encode(
    alt.X('month(date)'), alt.Y('temp_max')
)
mplaltair.convert(chart)

Actual outcome

A NotImplementedError is raised for anything that has a timeUnit.

Expected outcome

Any plot with a timeUnit transformation should work.

This plot should look something like this:

Possible solution and notes

• Manipulate the data in parse_chart.ChannelMetadata() so that the data reflects the timeUnit aggregation. (see parse_chart.ChannelMetadata._handle_timeUnit().)
• Also note that timeUnit transformations can also be applied separately from timeUnit transformations in the encoding channel.

Start with the reproduction using the Seattle weather dataset and inspect parse_chart.ChannelMetadata(), especially _handle_timeUnit(), where the issue identifies the current limitation. Check how timeUnit is represented in channel encoding and how separate timeUnit transformations are handled. Done means the reproduction and other timeUnit transformations convert without NotImplementedError and produce the expected time-based plot.