An external legend should carry the label from the chart, whether explicit or implicit (#607)

closes #605

Author: Fil

src/legends/ramp.js

@@ -3,7 +3,7 @@ import {interpolatePiecewise} from "../scales/quantitative.js";
 import {applyInlineStyles, maybeClassName} from "../style.js";
 
 export function legendRamp(color, {
-  label,
+  label = color.label,
   tickSize = 6,
   width = 240,
   height = 44 + tickSize,

test/output/colorLegendImplicitLabel.svg

@@ -204,6 +204,13 @@ export function colorLegendQuantileImplicit() {
   }).legend("color");
 }
 
+export function colorLegendImplicitLabel() {
+  return Plot.plot({
+    color: {scheme: "viridis"},
+    marks: [Plot.dot(d3.range(100).map(i => ({thing: i})), {fill: "thing"})]
+  }).legend("color");
+}
+
 export function colorLegendDiverging() {
   return Plot.legend({
     color: {