Add showForNull (or similar) to option.tooltip so tooltips appear for null/NaN values

[aldocarrolo]

Summary

Please add a parameter to option.tooltip that allows tooltips to be shown for data points whose value is null/NaN. This would enable users to surface useful context (e.g., “No data reported”, “Sensor offline”, or custom fallback text) when hovering or focusing points with missing values.

API Changes

Introduce a new boolean flag:

option.tooltip.showForNull = false; // default

Behavior:

• When showForNull: true, hovering/focusing a datum with value === null (or Number.isNaN(value) === true) should still trigger the tooltip.
• The formatter should be called with the usual params, plus a hint that the datum is null. For example:
  • params.value remains null
  • Optionally add params.isNull === true (or expose via params.data == null), so formatters can branch cleanly.
• This should work consistently across series types (line, bar, scatter, heatmap, etc.) and with connectNulls: true/false.
• Keyboard focus and touch interactions should mirror hover behavior for accessibility.

Intended Use Case

const option = {
  tooltip: {
    show: true,
    showForNull: true,
    formatter: (params) => {
      const label = params.axisValueLabel ?? params.name ?? '';
      if (params.value == null || Number.isNaN(params.value)) {
        return `No data for <b>${label}</b>`;
      }
      return `${params.seriesName}: <b>${params.value}</b>`;
    }
  },
  xAxis: { type: 'category', data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri'] },
  yAxis: { type: 'value' },
  series: [{
    name: 'Visitors',
    type: 'line',
    connectNulls: false,
    data: [820, null, 901, null, 1290]
  }]
};

Expected results

Hovering over Tue and Thu shows a tooltip like “No data for Tue/Thu” instead of nothing.

Use cases

• Time series with gaps (e.g., telemetry outages, holidays, late-arriving data).
• Dashboards that must explain why a point is missing rather than silently omitting it.
• Accessibility: screen-reader users and keyboard navigation benefit from explicit messaging for nulls.
• Auditing: clarify whether gaps are intentional (e.g., not applicable) or errors (e.g., failed ingestion).