Keep NaN values in data series

While it is important to maintain average, max, min values, this
should be done at the chart level and not at the data level, as these
values cannot be removed from the dataset arbitrarily.

app/assets/javascripts/helpers/monitor_helper.js

@@ -65,18 +65,10 @@ const getSeriesLabel = (queryLabel, metricAttributes) => {
  */
 // eslint-disable-next-line import/prefer-default-export
 export const makeDataSeries = (queryResults, defaultConfig) =>
-  queryResults
-    .map(result => {
-      // NaN values may disrupt avg., max. & min. calculations in the legend, filter them out
-      const data = result.values.filter(([, value]) => !Number.isNaN(value));
-      if (!data.length) {
-        return null;
-      }
-      const series = { data };
+  queryResults.map(result => {
     return {
       ...defaultConfig,
-        ...series,
+      data: result.values,
       name: getSeriesLabel(defaultConfig.name, result.metric),
     };
-    })
-    .filter(series => series !== null);
+  });

changelogs/unreleased/223790-nan-or-null-values-should-not-be-removed.yml

+---
+title: Stop removing NaN values from monitoring data series
+merge_request: 35086
+author:
+type: changed

spec/frontend/helpers/monitor_helper_spec.js

@@ -33,15 +33,6 @@ describe('monitor helper', () => {
       ]);
     });
 
-    it('excludes NaN values', () => {
-      expect(
-        monitorHelper.makeDataSeries(
-          data({ metric: {}, values: [[1, 1], [2, NaN]] }),
-          defaultConfig,
-        ),
-      ).toEqual([{ ...expectedDataSeries[0], data: [[1, 1]] }]);
-    });
-
     it('updates series name from templates', () => {
       const config = {
         ...defaultConfig,

spec/frontend/monitoring/store/utils_spec.js

@@ -815,6 +815,50 @@ describe('normalizeQueryResponseData', () => {
       },
     ]);
   });
+
+  it('processes a scalar result with a NaN result', () => {
+    // Queries may return "NaN" string values.
+    // e.g. when Prometheus cannot find a metric the query
+    // `scalar(does_not_exist)` will return a "NaN" value.
+
+    const mockScalar = {
+      resultType: 'scalar',
+      result: [1435781451.781, 'NaN'],
+    };
+
+    expect(normalizeQueryResponseData(mockScalar)).toEqual([
+      {
+        metric: {},
+        value: ['2015-07-01T20:10:51.781Z', NaN],
+        values: [['2015-07-01T20:10:51.781Z', NaN]],
+      },
+    ]);
+  });
+
+  it('processes a matrix result with a "NaN" value', () => {
+    // Queries may return "NaN" string values.
+    const mockMatrix = {
+      resultType: 'matrix',
+      result: [
+        {
+          metric: {
+            __name__: 'up',
+            job: 'prometheus',
+            instance: 'localhost:9090',
+          },
+          values: [[1435781430.781, '1'], [1435781460.781, 'NaN']],
+        },
+      ],
+    };
+
+    expect(normalizeQueryResponseData(mockMatrix)).toEqual([
+      {
+        metric: { __name__: 'up', instance: 'localhost:9090', job: 'prometheus' },
+        value: ['2015-07-01T20:11:00.781Z', NaN],
+        values: [['2015-07-01T20:10:30.781Z', 1], ['2015-07-01T20:11:00.781Z', NaN]],
+      },
+    ]);
+  });
 });
 
 describe('normalizeCustomDashboardPath', () => {