Make :field optional in vega lite spec

[rfhayashi]

:field can be omitted when doing things like histograms (e.g. https://vega.github.io/vega-lite/docs/bin.html#histogram). At the moment is possible to workaround this by setting :field to a random value, but it took me a long time to figure out why the chart didn't render. I'm not sure if :field should also be made optional for :x.

[rfhayashi]

@BrianChevalier Could you review this?

[BrianChevalier]

@rfhayashi Sorry to hear about the lost time 😞 This seems like a reasonable change, but I'm not sure if it will cause false positives (trying to render Vega-lite when map is not actually Vega-lite). Ideally we would have comprehensive 'correct' specs for Vega-lite. @djblue thoughts on this?

[rfhayashi]

@BrianChevalier This is an amazing thing that you did. I'm loving it. More than worth that lost time. My take is that if a map has :data, :mark and :enconding, it should be treated as vega and the viewer show an error if that is invalid. One can always change the viewer, so I guess that would be a good trade-off.

[djblue]

@BrianChevalier thanks for the feedback and I think making things easier is reasonable. Thanks @rfhayashi for the PR!