Choropleth keep d3.geoAlbers the default projection but deprecate it

[gordonwoodhull]

In the future we want geoChoroplethChart to default to the null (identity) projection since it is built on d3.geoPath which so defaults.

However we have the opportunity to get there in a backward-compatible way, with a deprecation warning.

Here's one way to get there:

• add a member _projection, which defaults to undefined
• at the start of doRender and doRedraw (maybe in a helper function)

if(_projection === undefined) { 
    console.warn('choropleth projection default of geoAlbers is deprecated');
    _geoPath.projection(d3.geoAlbers());
} else {
     _geoPath.projection(_projection);
}

• geoChoroplethChart.projection becomes a regular getter/setter of _projection (note getter is currently broken)

[gordonwoodhull]

In the future, we just default _projection to null and get rid of the if(_projection stuff.

[kum-deepak]

Good idea!

[kum-deepak]

Going after this one next :)

From https://github.com/d3/d3/blob/master/CHANGES.md

The default projection for d3.geoPath is now null rather than d3.geoAlbersUsa; a null projection is used with pre-projected geometry and is typically faster to render.

Also going through https://bl.ocks.org/mbostock/5557726

It seems that null is a valid value for projection now. So, will program accordingly.

[gordonwoodhull]

Great! Yes let's plan for null being the default in the future. If people want it now they can set it explicitly. But undefined for now means "no value set", which we'll default to Albers.

[gordonwoodhull]

fixed by #1382