channel method
Sets any channel to a column by name.
The named channel becomes the cursor, so the next aggregate, title or
type applies to it.
name The channel name, as Vega-Lite spells it.
field The column name.
Returns This mark, so calls chain.
Implementation
VlChartMark channel(String name, String field) {
VlJson ch = VlJson.objectValue();
ch.setMember("field", VlJson.stringValue(field));
this.enc.setMember(name, ch);
cursor = name;
return this;
}