diff --git a/lib/graphlib.php b/lib/graphlib.php
index c06db479b79fe..0eebb05728a42 100644
--- a/lib/graphlib.php
+++ b/lib/graphlib.php
@@ -537,7 +537,10 @@ function draw_zero_axis_left() {
       $colour = $this->parameter['zero_axis'];
       if ($colour == 'none') return;
       // draw zero axis on left hand side
-      $this->calculated['zero_axis'] = round($this->calculated['boundary_box']['top']  + ($this->calculated['y_axis_left']['max'] * $this->calculated['y_axis_left']['factor']));
+      $this->calculated['zero_axis'] = (int) round(
+        $this->calculated['boundary_box']['top'] +
+        ($this->calculated['y_axis_left']['max'] * $this->calculated['y_axis_left']['factor'])
+      );
       ImageLine($this->image, $this->calculated['boundary_box']['left'], $this->calculated['zero_axis'], $this->calculated['boundary_box']['right'], $this->calculated['zero_axis'], $this->colour[$colour]);
     }
 
@@ -545,7 +548,10 @@ function draw_zero_axis_right() {
       $colour = $this->parameter['zero_axis'];
       if ($colour == 'none') return;
       // draw zero axis on right hand side
-      $this->calculated['zero_axis'] = round($this->calculated['boundary_box']['top']  + ($this->calculated['y_axis_right']['max'] * $this->calculated['y_axis_right']['factor']));
+      $this->calculated['zero_axis'] = (int) round(
+        $this->calculated['boundary_box']['top'] +
+        ($this->calculated['y_axis_right']['max'] * $this->calculated['y_axis_right']['factor'])
+      );
       ImageLine($this->image, $this->calculated['boundary_box']['left'], $this->calculated['zero_axis'], $this->calculated['boundary_box']['right'], $this->calculated['zero_axis'], $this->colour[$colour]);
     }
 
@@ -691,10 +697,10 @@ function draw_y_axis() {
           if (!$this->calculated['y_axis_left']['has_data'] && $yGrid != 'none') { // draw grid if not drawn already (above)
             switch ($yGrid) {
               case 'line':
-                ImageLine($this->image, round($gridLeft), round($tickY), round($gridRight), round($tickY), $gridColour);
+                ImageLine($this->image, (int) round($gridLeft), (int) round($tickY), (int) round($gridRight), (int) round($tickY), $gridColour);
                 break;
                case 'dash':
-                $this->image_dashed_line($this->image, round($gridLeft), round($tickY), round($gridRight), round($tickY), $gridColour); // Moodle
+                $this->image_dashed_line($this->image, (int) round($gridLeft), (int) round($tickY), (int) round($gridRight), (int) round($tickY), $gridColour); // Moodle
                 break;
             }
           }
@@ -702,7 +708,7 @@ function draw_y_axis() {
           if ($this->parameter['y_axis_text_right'] && !($set % $this->parameter['y_axis_text_right'])) { // test if tick should be displayed
             // draw tick
             if ($tickColour != 'none')
-              ImageLine($this->image, round($tickLeft), round($tickY), round($tickRight), round($tickY), $tickColour);
+              ImageLine($this->image, (int) round($tickLeft), (int) round($tickY), (int) round($tickRight), (int) round($tickY), $tickColour);
 
             // draw axis text...
             $coords = array('x' => $textLeft, 'y' => $tickY, 'reference' => $reference);
@@ -767,12 +773,12 @@ function init_data() {
 
             if (isset($this->y_format[$set]['y_axis']) && $this->y_format[$set]['y_axis'] == 'right') {
               $this->calculated['y_plot'][$set][$index] =
-                round(($this->y_data[$set][$index] - $this->calculated['y_axis_right']['min'])
+                (int) round(($this->y_data[$set][$index] - $this->calculated['y_axis_right']['min'])
                   * $this->calculated['y_axis_right']['factor']);
             } else {
               //print "$set $index<br />";
               $this->calculated['y_plot'][$set][$index] =
-                round(($this->y_data[$set][$index] - $this->calculated['y_axis_left']['min'])
+                (int) round(($this->y_data[$set][$index] - $this->calculated['y_axis_left']['min'])
                   * $this->calculated['y_axis_left']['factor']);
             }
 
@@ -812,7 +818,7 @@ function init_x_ticks() {
         // x tick value
         $this->calculated['x_axis']['tick_x'][$set] = $tickX;
         // if num ticks is auto then x plot value is same as x  tick
-        if ($this->parameter['x_axis_gridlines'] == 'auto') $this->calculated['x_plot'][$set] = round($tickX);
+        if ($this->parameter['x_axis_gridlines'] == 'auto') $this->calculated['x_plot'][$set] = (int) round($tickX);
         //print $this->calculated['x_plot'][$set].'<br />';
         $tickX += $xStep;
       }
@@ -1179,7 +1185,7 @@ function find_range($data, $min, $max, $resolution) {
         else $factor = pow(10, (floor(log10(abs($max))) - $resolution) );
       }
       if ($factor > 0.1) { // To avoid some wierd rounding errors (Moodle)
-        $factor = round($factor * 1000.0) / 1000.0; // To avoid some wierd rounding errors (Moodle)
+        $factor = (int) round($factor * 1000.0) / 1000.0; // To avoid some wierd rounding errors (Moodle)
       } // To avoid some wierd rounding errors (Moodle)
 
       $max = $factor * @ceil($max / $factor);
@@ -1671,7 +1677,7 @@ function bar($x, $y, $type, $size, $colour, $offset, $index, $yoffset) {
 
         // Moodle addition, plus the function parameter yoffset
         if ($yoffset) {                                           // Moodle
-            $yoffset = $yoffset - round(($bottom - $v) / 2.0);    // Moodle
+            $yoffset = $yoffset - (int) round(($bottom - $v) / 2.0);    // Moodle
             $bottom -= $yoffset;                                  // Moodle
             $v      -= $yoffset;                                  // Moodle
         }                                                         // Moodle
@@ -1779,11 +1785,11 @@ function draw_brush_line($x0, $y0, $x1, $y1, $size, $type, $colour) {
         $t += $y0;
         $dx = ($dx < 0) ? -1 : 1;
         $m *= $dx;
-        while (round($x0) != round($x1)) {
+        while ((int) round($x0) != (int) round($x1)) {
           if (!$watchdog--) break;
           $x0 += $dx; // step to next x value
           $t += $m;   // add slope to y value
-          $y = round($t);
+          $y = (int) round($t);
           //$this->dbug("x0=$x0, x1=$x1, y=$y watchdog=$watchdog");
           $this->draw_brush($x0, $y, $size, $type, $colour);
 
@@ -1794,11 +1800,11 @@ function draw_brush_line($x0, $y0, $x1, $y1, $size, $type, $colour) {
         $t += $x0;
         $dy = ($dy < 0) ? -1 : 1;
         $m *= $dy;
-        while (round($y0) != round($y1)) {
+        while ((int) round($y0) != (int) round($y1)) {
           if (!$watchdog--) break;
           $y0 += $dy; // step to next y value
           $t += $m;   // add slope to x value
-          $x = round($t);
+          $x = (int) round($t);
           //$this->dbug("x=$x, y0=$y0, y1=$y1 watchdog=$watchdog");
           $this->draw_brush($x, $y0, $size, $type, $colour);
 
@@ -1807,9 +1813,9 @@ function draw_brush_line($x0, $y0, $x1, $y1, $size, $type, $colour) {
     }
 
     function draw_brush($x, $y, $size, $type, $colour) {
-      $x = round($x);
-      $y = round($y);
-      $half = round($size / 2);
+      $x = (int) round($x);
+      $y = (int) round($y);
+      $half = (int) round($size / 2);
       switch ($type) {
         case 'circle':
           ImageArc($this->image, $x, $y, $size, $size, 0, 360, $this->colour[$colour]);
diff --git a/lib/tests/graphlib_test.php b/lib/tests/graphlib_test.php
index a8838cebc64d1..f2afe31acb7ea 100644
--- a/lib/tests/graphlib_test.php
+++ b/lib/tests/graphlib_test.php
@@ -53,17 +53,35 @@ public function create_data(): array {
                         -1, -1, -1, -1, -1, -1
                     ],
                     'strpreferred' => 'Preferred',
+                    'strimagine' => 'Imagine',
+                    'buckets3' => [
+                        1,
+                        2.75,
+                        0.5,
+                        3.5,
+                        1.25,
+                        3,
+                    ],
                     'stdev1' => [
                         0.82915619758885, 1.1180339887499, 1.1180339887499, 1.1180339887499, 1.1180339887499, 1.1180339887499
                     ],
                     'stdev2' => [
                         0, 0, 0, 0, 0, 0
                     ],
+                    'stdev3' => [
+                        0.92915619758885,
+                        2.1180339887499,
+                        2.1180339887499,
+                        2.1180339887499,
+                        2.1180339887499,
+                        2.1180339887499,
+                    ],
                     'options' => [
                         'Almost never', 'Seldom', 'Sometimes', 'Often', 'Almost always'
                     ],
                     'maxbuckets1' => 2.5,
-                    'maxbuckets2' => -1
+                    'maxbuckets2' => -1,
+                    'maxbuckets3' => 3.5,
                 ]
             ]
         ];
@@ -81,6 +99,7 @@ public function test_graphlib($mock) {
         $graph = new \graph(300, 200);
         ob_start();
         $graph->parameter['title'] = strip_tags(format_string($mock['survey_name'], true));
+        $graph->parameter['zero_axis'] = 'black';
         $graph->x_data = $mock['names'];
         $graph->y_data['answers1'] = $mock['buckets1'];
         $graph->y_format['answers1'] = array('colour' => 'ltblue', 'line' => 'line', 'point' => 'square',
@@ -88,24 +107,53 @@ public function test_graphlib($mock) {
         $graph->y_data['answers2'] = $mock['buckets2'];
         $graph->y_format['answers2'] = array('colour' => 'ltorange', 'line' => 'line', 'point' => 'square',
                 'shadow_offset' => 4, 'legend' => $mock['strpreferred']);
+        $graph->y_data['answers3'] = $mock['buckets3'];
+        $graph->y_format['answers3'] = [
+            'colour' => 'ltred',
+            'line' => 'brush',
+            'point' => 'square',
+            'shadow_offset' => 4,
+            'legend' => $mock['stractual'],
+        ];
         $graph->y_data['stdev1'] = $mock['stdev1'];
         $graph->y_format['stdev1'] = array('colour' => 'ltltblue', 'bar' => 'fill',
                 'shadow_offset' => '4', 'legend' => 'none', 'bar_size' => 0.3);
         $graph->y_data['stdev2'] = $mock['stdev2'];
         $graph->y_format['stdev2'] = array('colour' => 'ltltorange', 'bar' => 'fill',
                 'shadow_offset' => '4', 'legend' => 'none', 'bar_size' => 0.2);
+        $graph->y_data['stdev3'] = $mock['stdev3'];
+        $graph->y_format['stdev3'] = [
+            'colour' => 'ltred',
+            'bar' => 'fill',
+            'shadow_offset' => '4',
+            'legend' => 'none',
+            'bar_size' => 0.2,
+        ];
         $graph->offset_relation['stdev1'] = 'answers1';
         $graph->offset_relation['stdev2'] = 'answers2';
+        $graph->offset_relation['stdev3'] = 'answers3';
         $graph->parameter['legend'] = 'outside-top';
         $graph->parameter['legend_border'] = 'black';
         $graph->parameter['legend_offset'] = 4;
         $graph->y_tick_labels = $mock['options'];
-        if (($mock['maxbuckets1'] > 0.0) && ($mock['maxbuckets2'] > 0.0)) {
-            $graph->y_order = array('stdev1', 'answers1', 'stdev2', 'answers2');
+        if (($mock['maxbuckets1'] > 0.0) && ($mock['maxbuckets2'] > 0.0) && ($mock['maxbuckets3'] > 0.0)) {
+            $graph->y_order = [
+                'stdev1',
+                'answers1',
+                'stdev2',
+                'answers2',
+                'stdev3',
+                'answers3',
+            ];
         } else if ($mock['maxbuckets1'] > 0.0) {
             $graph->y_order = array('stdev1', 'answers1');
-        } else {
+        } else if ($mock['maxbuckets2'] > 0.0) {
             $graph->y_order = array('stdev2', 'answers2');
+        } else {
+            $graph->y_order = [
+                'stdev3',
+                'answers3',
+            ];
         }
         $graph->parameter['y_max_left'] = 4;
         $graph->parameter['y_axis_gridlines'] = 5;