Improve syntax for argmin/max

[kanitw]

Right now we need something like

alt.Y(aggregate=alt.ArgmaxDef(argmax=<f>), field=..., type=...)

to make the following in Vega-Lite:

{aggregate: {argmax: <f>}, field: ..., type: ...}

It would be more natural to allow:

alt.Y(aggregate=alt.Argmax(<f>), field=..., type=...)

Same for argmin. :)

[jakevdp]

That syntax should work currently without the keyword

>>> import altair as alt
>>> alt.X('x:Q', aggregate=alt.ArgminDef('y')).to_dict()
{'aggregate': {'argmin': 'y'}, 'field': 'x', 'type': 'quantitative'}

[kanitw]

Ah i see. The only question is whether we should cut ‘Def’ suffix to make it more reader friendly. If you agree, i can fix it from Vega-Lite schema as well.