Avoid concatenating categorical axis values if empty
@@ -1180,7 +1180,9 @@ class OverlayPlot(GenericOverlayPlot, LegendPlot):
xfs, yfs = sp._get_factors(el)
xfactors.append(xfs)
yfactors.append(yfs)
+ if xfactors:
xfactors = np.concatenate(xfactors)
+ if yfactors:
yfactors = np.concatenate(yfactors)
return util.unique_array(xfactors), util.unique_array(yfactors)
