Refine data generator support

[jheer]

Vega-Lite supports data generators for geo spheres, geo graticules, and numeric sequences: https://vega.github.io/vega-lite/docs/data.html#data-generators

Altair includes auto-generated classes (e.g., alt.GraticuleGenerator, alt.GraticuleParams) corresponding to these features. However, passing a generator class as the data for a Chart can result in warning output:

UserWarning: data of type <class ‘altair.vegalite.v3.schema.core.SphereGenerator’> not recognized
warnings.warn(“data of type {} not recognized”.format(type(data)))

Moreover, code like alt.GraticuleGenerator(graticule=alt.GraticuleParams(...)) feels unnecessarily verbose. Perhaps a more concise syntax would be helpful? For example, merging the generator and params into a single call... For comparison, the Vega-Lite JavaScript API includes top-level methods for generators (vl.graticule()); https://observablehq.com/@vega/vega-lite-cartographic-projections provides an example of their use.

[jakevdp]

Great suggestion! I think something like alt.graticule() or alt.data.graticule() would make sense.

What version were these added to Vega-Lite?