Fix incorrect scaling of data_fraction column in forward_backward_con…

…vergence (issue #350, PR #355)

* fix #350 
* The data_fraction column in forward_backward_convergence was scaled by the unit converter 
   in ABFE workflow. This fix ensures that the column values (ranging from 0 to 1) are never 
   accidentally treated as having an energy unit.
* add test
* update CHANGES

CHANGES

@@ -30,6 +30,10 @@ Enhancements
     (issue #337, PR #338)
   - ValueError issued when `df` and `series` for `statistical_inefficiency`
     doesn't have the same length (issue #337, PR #338)
+
+Fixes
+  - data_fraction column in workflow.convergence won't be affected by the
+    unit conversion (issue #350, PR#319).
 
 
 22/06/2023 xiki-tempula

src/alchemlyb/tests/test_workflow_ABFE.py

@@ -368,6 +368,12 @@ def test_ti_convergence(self, workflow, monkeypatch):
         workflow.check_convergence(10, estimator="TI")
         assert len(workflow.convergence) == 10
 
+    def test_preserve_unit(self, workflow, monkeypatch):
+        monkeypatch.setattr(workflow, "convergence", None)
+        monkeypatch.setattr(workflow, "units", "kcal/mol")
+        workflow.check_convergence(2, estimator="TI")
+        assert np.allclose(workflow.convergence["data_fraction"], [0.5, 1.0])
+
     def test_unprocessed_dhdl(self, workflow, monkeypatch):
         monkeypatch.setattr(workflow, "dHdl_sample_list", None)
         monkeypatch.setattr(workflow, "convergence", None)

src/alchemlyb/workflows/abfe.py

@@ -801,7 +801,10 @@ def check_convergence(
             logger.error(msg)
             raise ValueError(msg)
 
-        self.convergence = get_unit_converter(self.units)(convergence)
+        unit_converted_convergence = get_unit_converter(self.units)(convergence)
+        # Otherwise the data_fraction column is converted as well.
+        unit_converted_convergence["data_fraction"] = convergence["data_fraction"]
+        self.convergence = unit_converted_convergence
 
         logger.info(f"Plot convergence analysis to {dF_t} under {self.out}.")