Warn but don't crash on missing key in differ

Validation/src/Validation/_differ.py

@@ -7,6 +7,7 @@
 # external dependencies
 import matplotlib.pyplot as plt
 import matplotlib
+import uproot
 # us
 from ._file import File
 
@@ -110,7 +111,11 @@ def plot1d(self, column, xlabel,
         ax = fig.subplots()
 
         for f in self.files :
-            weights, bins, patches = f.plot1d(ax, column, **hist_kwargs)
+            try:
+                weights, bins, patches = f.plot1d(ax, column, **hist_kwargs)
+            except uproot.KeyInFileError:
+                f.log.warn(f"Key {column} doesn't exist in {self}, skipping")
+                continue
 
         ax.set_xlabel(xlabel)
         ax.set_ylabel(ylabel)