[FIX] chart: trend line tooltip for scatter/line charts

For scatter/line chart whose labels were numbers, the trend line
tooltip was showing something like `(15, 9)`. 9 was the
correct Y value, but the 15 was referring to the fact that it was the
15th point in the generated trend line. Which made no sense to the user.

This commit changes the tooltip to show only the Y value.

closes #5268

Task: 4279972
Signed-off-by: Rémi Rahir (rar) <[email protected]>

src/helpers/figures/charts/chart_common_line_scatter.ts

@@ -315,7 +315,10 @@ export function createLineOrScatterChartRuntime(
       formatValue(value, { format: labelFormat, locale });
     config.options.plugins!.tooltip!.callbacks!.label = (tooltipItem) => {
       const dataSetPoint = dataSetsValues[tooltipItem.datasetIndex!].data![tooltipItem.dataIndex!];
-      let label: string | number = tooltipItem.label || labelValues.values[tooltipItem.dataIndex!];
+      let label: string | number =
+        tooltipItem.dataset.xAxisID === TREND_LINE_XAXIS_ID
+          ? ""
+          : tooltipItem.label || labelValues.values[tooltipItem.dataIndex!];
       if (isNumber(label, locale)) {
         label = toNumber(label, locale);
       }

tests/figures/chart/chart_plugin.test.ts

@@ -2128,6 +2128,25 @@ describe("Chart design configuration", () => {
     expect(label).toBe("Dataset 1: (500%, $6,000.00)");
   });
 
+  test("scatter chart trend line tooltip label", () => {
+    setGrid(model, { A1: "1", A2: "2", B1: "12", B2: "15" });
+
+    createChart(
+      model,
+      {
+        labelRange: "A1:A2",
+        dataSets: [{ dataRange: "B1:B2", trend: { type: "polynomial", order: 1, display: true } }],
+        type: "scatter",
+        dataSetsHaveTitle: false,
+      },
+      "1"
+    );
+    const chart = model.getters.getChartRuntime("1") as ScatterChartRuntime;
+    const label = getTooltipLabel(chart, 1, 0);
+
+    expect(label).toBe("Trend line for Series 1: 12");
+  });
+
   test.each(["line", "scatter", "bar", "combo"] as const)(
     "%s chart correctly use right axis if set up in definition",
     (chartType) => {