avoid crash in alchemlyb.visualisation.plot_convergence with NaN (#319)

- fix #318
- only plot final error estimate in  alchemlyb.visualisation.plot_convergence if it is not NaN

Co-authored-by: Zhiyi Wu <zwu@exscientia.co.uk>

CHANGES

@@ -13,6 +13,15 @@ The rules for this file:
   * release numbers follow "Semantic Versioning" https://semver.org
 
 ------------------------------------------------------------------------------
+*/*/2023 xiki-tempula
+
+  * 2.1.0
+
+Fixes
+  - Fix the case where visualisation.plot_convergence would fail when the final
+   error is NaN (issue #318, PR#317).
+
+
 06/04/2023 xiki-tempula
 
   * 2.0.1

src/alchemlyb/tests/test_visualisation.py

@@ -202,6 +202,22 @@ def test_plot_convergence(gmx_benzene_Coulomb_u_nk):
     plt.close(ax.figure)
 
 
+def test_plot_convergence_final_nan():
+    """Test the case where the Error of the final estimate is NaN."""
+    df = pd.DataFrame(
+        data={
+            "Forward": [1, 2],
+            "Forward_Error": [np.nan, np.nan],
+            "Backward": [1, 2],
+            "Backward_Error": [np.nan, np.nan],
+        }
+    )
+    df.attrs = {"temperature": 300, "energy_unit": "kT"}
+    ax = plot_convergence(df)
+    assert isinstance(ax, matplotlib.axes.Axes)
+    plt.close(ax.figure)
+
+
 class Test_Units:
     @staticmethod
     @pytest.fixture()

src/alchemlyb/visualisation/convergence.py

@@ -93,13 +93,14 @@ def plot_convergence(dataframe, units=None, final_error=None, ax=None):
     if final_error is None:
         final_error = backward_error[-1]
 
-    line0 = ax.fill_between(
-        [0, 1],
-        backward[-1] - final_error,
-        backward[-1] + final_error,
-        color="#D2B9D3",
-        zorder=1,
-    )
+    if np.isfinite(backward[-1]) and np.isfinite(final_error):
+        line0 = ax.fill_between(
+            [0, 1],
+            backward[-1] - final_error,
+            backward[-1] + final_error,
+            color="#D2B9D3",
+            zorder=1,
+        )
     line1 = ax.errorbar(
         f_ts,
         forward,