valueString method
Sets a channel to a constant string rather than to a column.
This is how a mark is painted a fixed colour: .valueString("color" "#c00").
The channel setters name columns and never constants, because a .color(…)
that guessed between the two would draw a chart nobody asked for.
channel The channel name.
value The constant.
Returns This mark, so calls chain.
See also valueNumber.
Implementation
VlChartMark valueString(String channel, String value) {
VlJson ch = VlJson.objectValue();
ch.setMember("value", VlJson.stringValue(value));
this.enc.setMember(channel, ch);
cursor = channel;
return this;
}