MDL-60632 mod_quiz: Fix chart rendering fail in quiz report

If a quiz activity has negative grading enabled for incorrect answers,
students can achieve overall negative grades in this quiz. If at least
one student achieves a negative grade, the chart at the bottom of the
quiz 'Results' tab will fail to display. This patch add a search for
negative results, removes them from the band below 0 and adds them to
the 0 band. This will make the chart render correctly again.

Co-authored-by: Susana Leitão <sleitao@uporto.pt>
Co-authored-by: Matthias Opitz <m.opitz@ucl.ac.uk>

Removed whitespace at end of lines

mod/quiz/report/reportlib.php

@@ -248,6 +248,14 @@ function quiz_report_grade_bands($bandwidth, $bands, $quizid, \core\dml\sql_join
     $data[$bands - 1] += $data[$bands];
     unset($data[$bands]);
 
+    // See MDL-60632. When a quiz participant achieves an overall negative grade the chart fails to render.
+    foreach ($data as $databand => $datanum) {
+        if ($databand < 0) {
+            $data["0"] += $datanum; // Add to band 0.
+            unset($data[$databand]); // Remove entry below 0.
+        }
+    }
+
     return $data;
 }
 

mod/quiz/tests/reportlib_test.php

@@ -158,4 +158,37 @@ public function test_quiz_report_qm_filter_select_first_last_best() {
         $bestattempt = reset($bestattempt);
         $this->assertEquals(2, $bestattempt->attempt);
     }
+
+    public function test_quiz_results_never_below_zero() {
+        global $DB;
+        $this->resetAfterTest();
+
+        $quizid = 7;
+        $fakegrade = new \stdClass();
+        $fakegrade->quiz = $quizid;
+
+        // Have 5 test grades.
+        $fakegrade->userid = 10;
+        $fakegrade->grade = 6.66667;
+        $DB->insert_record('quiz_grades', $fakegrade);
+
+        $fakegrade->userid = 11;
+        $fakegrade->grade = -2.86;
+        $DB->insert_record('quiz_grades', $fakegrade);
+
+        $fakegrade->userid = 12;
+        $fakegrade->grade = 10.0;
+        $DB->insert_record('quiz_grades', $fakegrade);
+
+        $fakegrade->userid = 13;
+        $fakegrade->grade = -5.0;
+        $DB->insert_record('quiz_grades', $fakegrade);
+
+        $fakegrade->userid = 14;
+        $fakegrade->grade = 33.33333;
+        $DB->insert_record('quiz_grades', $fakegrade);
+
+        $data = quiz_report_grade_bands(5, 20, $quizid);
+        $this->assertGreaterThanOrEqual(0, min(array_keys($data)));
+    }
 }