add field to scale access, see vega/vega#440
add field to scale access, see vega/vega#440
vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite,vega/vega-lite,vega/vega-lite,uwdata/vega-lite,uwdata/vega-lite,vega/vega-lite