--- a/modules/charts_chartjs/src/Plugin/chart/Library/Chartjs.php
+++ b/modules/charts_chartjs/src/Plugin/chart/Library/Chartjs.php
@@ -507,7 +507,12 @@
           $categories = $element[$child]['#labels'];
           break;
         }
-        $categories = is_array($element[$child]['#labels']) ? array_map('strip_tags', $element[$child]['#labels']) : [];
+        $categories = is_array($element[$child]['#labels']) ? array_map(function ($label) {
+          // Chart.js supports an array of strings as a single tick's label,
+          // rendering each item on its own line; strip_tags() can't take an
+          // array, so recurse into it instead of calling strip_tags() directly.
+          return is_array($label) ? array_map('strip_tags', $label) : strip_tags($label);
+        }, $element[$child]['#labels']) : [];
       }
       if (in_array($element['#chart_type'], $this->getPieStyleTypes())
         && $type !== 'chart_xaxis') {
