diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a64ac217acc1326b8320e36f738b7dfd67673d1c..817d7e86f91e4914d054af17c9519b097e661ae6 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -152,6 +152,8 @@ tests (win - pip) [R]:
   rules:
     - *regression-rules
   needs: []
+  variables:
+    MPLBACKEND: Agg  # non-interactive backend for matplotlib
   before_script:
     - python -m venv .venv
     - .venv\Scripts\Activate.ps1