Add timeUnit support

[kdorr]

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.