Add binning support

[kdorr]

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

Code for reproduction

from vega_datasets import data
seattle = data.seattle_weather()
seattle.head()

Binning color in a scatter plot:

chart = alt.Chart(seattle).mark_point().encode(
    alt.X('wind'), alt.Y('temp_max'), alt.Color('temp_min', bin=alt.Bin(step=10))
)
mplaltair.convert(chart)

Binning in a bar chart:

chart = alt.Chart(seattle).mark_bar().encode(
    alt.X('wind', bin=True), alt.Y('count()')
)
mplaltair.convert(chart)

Actual outcome

In a chart that would work without binning, binning fails silently and the chart is converted as if binning is always false.

Binning color in a scatter plot:

Binning in a bar chart:

raises NotImplementedError until bar charts and aggregates are working.

Expected outcome
Converting a scatter plot with binned color should produce a chart that looks something like this:

Converting a binned bar chart should produce a chart that looks something like this:

Possible solution and notes

• Manipulate the data to be binned in parse_chart.ChannelMetadata().
• Binning is probably better to add when bar charts and aggregate functions are functional in mplaltair.
• Be aware that binning can happen in the encoding definition or in a transform.

For more info

• Altair's user guide on binning
• Vega-Lite documentation on binning