fix: handle splitted series with group value to 0 (#289)

The current implementation wrongly interpret a zero value in the colorValues list of a legend item as a single series. This cause that series to be named with the spec name or the spec id. Instead a splitted series with a group value of zero, should be treated in the same way as any other splitted series, using the group value as the series name

fix #288

src/chart_types/xy_chart/legend/legend.test.ts

@@ -216,5 +216,41 @@ describe('Legends', () => {
     expect(label).toBe('a - b');
     label = getSeriesColorLabel(['a', 'b'], false, spec2);
     expect(label).toBe('a - b');
+
+    label = getSeriesColorLabel([null], true);
+    expect(label).toBeUndefined();
+    label = getSeriesColorLabel([null], true, spec1);
+    expect(label).toBe('Spec 1 title');
+    label = getSeriesColorLabel([null], true, spec2);
+    expect(label).toBe('spec2');
+    label = getSeriesColorLabel([undefined], true);
+    expect(label).toBeUndefined();
+    label = getSeriesColorLabel([undefined], true, spec1);
+    expect(label).toBe('Spec 1 title');
+    label = getSeriesColorLabel([undefined], true, spec2);
+    expect(label).toBe('spec2');
+
+    label = getSeriesColorLabel([0], true);
+    expect(label).toBeUndefined();
+    label = getSeriesColorLabel([0], true, spec1);
+    expect(label).toBe('Spec 1 title');
+
+    label = getSeriesColorLabel([null], false);
+    expect(label).toBeUndefined();
+    label = getSeriesColorLabel([null], false, spec1);
+    expect(label).toBe('Spec 1 title');
+    label = getSeriesColorLabel([null], false, spec2);
+    expect(label).toBe('spec2');
+    label = getSeriesColorLabel([undefined], false);
+    expect(label).toBeUndefined();
+    label = getSeriesColorLabel([undefined], false, spec1);
+    expect(label).toBe('Spec 1 title');
+    label = getSeriesColorLabel([undefined], false, spec2);
+    expect(label).toBe('spec2');
+
+    label = getSeriesColorLabel([0], false);
+    expect(label).toBe('0');
+    label = getSeriesColorLabel([0], false, spec1);
+    expect(label).toBe('0');
   });
 });

src/chart_types/xy_chart/legend/legend.ts

@@ -71,8 +71,7 @@ export function getSeriesColorLabel(
   spec?: BasicSeriesSpec,
 ): string | undefined {
   let label = '';
-
-  if (hasSingleSeries || colorValues.length === 0 || !colorValues[0]) {
+  if (hasSingleSeries || colorValues.length === 0 || colorValues[0] == null) {
     if (!spec) {
       return;
     }