improve test coverage by testing for ValueError

Author: johan12345

lib/matplotlib/tests/test_axes.py

@@ -4123,6 +4123,10 @@ def test_axline_args():
     ax.set_yscale('log')
     with pytest.raises(TypeError):
         ax.axline((0, 0), slope=1)
+    ax.set_yscale('linear')
+    with pytest.raises(ValueError):
+        ax.axline((0, 0), (0, 0))  # two identical points are not allowed
+        plt.draw()
 
 
 @image_comparison(['vlines_basic', 'vlines_with_nan', 'vlines_masked'],