added a test case

tests/test_saveagr.py

@@ -0,0 +1,15 @@
+import unittest
+import matplotlib.pyplot as plt
+import tudplot
+tudplot.activate()
+
+class TestStringMethods(unittest.TestCase):
+
+    def test_saveagr(self):
+        plt.plot([1,2,4,4.9,3.2,1.2,0.5,0.9,1.5],'o-', label='Test' )
+        plt.xlabel("1000 K/T")
+        plt.ylabel(r"log($T_1$) in 1/s")
+        tudplot.saveagr('test.agr')
+
+if __name__ == '__main__':
+    unittest.main()