root writer: skip defining histogram bins for non-numeric y values

* Skip defining bins for non-numeric y values unless alphanumeric bins.
* Needed for ROOT export of https://www.hepdata.net/record/ins1392456

Signed-off-by: Graeme Watt <graeme.watt@durham.ac.uk>

hepdata_converter/writers/root_writer.py

@@ -40,11 +40,15 @@ def match(cls, independent_variables_map, dependent_variable):
 
     def _create_empty_hist(self, dependent_var_title, index, yval):
 
+        is_number_list = self.is_number_var(self.dependent_variable)
+
         xval = []
         for i in xrange(self.dim):
             xval.append([])
             i_var = self.independent_variables[i]['values']
-            for x in i_var:
+            for ix, x in enumerate(i_var):
+                if not is_number_list[ix] and 'labels' not in self.independent_variables[i]:
+                    continue  # skip defining bins for non-numeric y values unless alphanumeric bin labels are present
                 if x['low'] not in xval[i]:
                     xval[i].append(x['low'])
                 if x['high'] not in xval[i]:
@@ -235,7 +239,9 @@ def create_objects(self):
         for i in xrange(self.dim):
             xval.append([])
             i_var = self.independent_variables[i]['values']
-            for x in i_var:
+            for ix, x in enumerate(i_var):
+                if not is_number_list[ix] and 'labels' not in self.independent_variables[i]:
+                    continue  # skip defining bins for non-numeric y values unless alphanumeric bin labels are present
                 if x['low'] not in xval[i]:
                     xval[i].append(x['low'])
                 if x['high'] not in xval[i]: