diff --git a/pyproject.toml b/pyproject.toml
index bebf22b..685ccdf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -22,6 +22,7 @@ build-backend = "setuptools.build_meta"
         "numpy>=1.24.2",
         "libcosimpy>=0.0.2",
         "fmpy>=0.3.21",
+        "matplotlib>=3.7.1",
     ]
     #dynamic = ["version"]
 
diff --git a/tox.ini b/tox.ini
index b43f620..dd274ed 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 isolated_build = True
-envlist = py{39,310,311,312}-{linux,macos,windows}
+envlist = py{310,311,312}-{linux,macos,windows}
 # envlist = py{39,310,311,312}-{windows}
 # envlist = py{39,310,311,312}