Check for categorical data to histogram (#5540)

holoviews/operation/element.py

@@ -786,6 +786,8 @@ def _process(self, element, key=None):
                 hist_range = (0, 1)
             steps = self.p.num_bins + 1
             start, end = hist_range
+            if isinstance(start, str) or isinstance(end, str) or isinstance(steps, str):
+                raise ValueError("Categorical data found. Cannot create histogram from categorical data.")
             if is_datetime:
                 start, end = dt_to_int(start, 'ns'), dt_to_int(end, 'ns')
             if self.p.log:

holoviews/tests/operation/test_operation.py

@@ -1,5 +1,6 @@
 import datetime as dt
 from unittest import skipIf
+import pytest
 
 import numpy as np
 import pandas as pd
@@ -256,6 +257,12 @@ def test_histogram_operation_pd_period(self):
         hist = Histogram(hist_data, kdims='Date', vdims=('Date_frequency', 'Frequency'))
         self.assertEqual(op_hist, hist)
 
+    def test_histogram_categorical(self):
+        series = Dataset(pd.Series(['A', 'B', 'C']))
+        kwargs = {'bin_range': ('A', 'C'), 'normed': False, 'cumulative': False, 'num_bins': 3}
+        with pytest.raises(ValueError):
+            histogram(series, **kwargs)
+
     def test_points_histogram_weighted(self):
         points = Points([float(i) for i in range(10)])
         op_hist = histogram(points, num_bins=3, weight_dimension='y', normed=True)