GeezerVison

Summary

px.histogram() crashes with TypeError: unsupported operand type(s) for +: 'NoneType' and 'str' when called with no data arguments, while other chart types like px.scatter(), px.bar(), and px.pie() handle empty invocations correctly.

Root Cause

The histogram-specific hover label path in _core.py unconditionally adds an entry to mapping_labels even when no column is specified. When no data is provided, attr_label is None (from get_decorated_label(args, None, 'x')), resulting in a {None: '%{x}'} entry. The hover template list comprehension then fails:

hover_lines = [k + "=" + v for k, v in mapping_labels_copy.items()]
#              None + "=" + ...  → TypeError

Fix

Skip adding to mapping_labels when attr_label is None, consistent with how other chart types handle empty invocations (they never reach the mapping_labels code path since their attrs check attr_value is not None first).

Test

Added test_empty_histogram to verify px.histogram() returns a valid figure without raising.

Fixes #5534