code: def test_save_png():
      chart = make_histogram()
      f = BytesIO()
      vf.save_png(chart, f)
      png = f.getvalue()
      assert png.startswith(b"\x89PNG")
  
file_path: vegafusion/python/vegafusion/tests/test_save.py
imports: import altair
  from vega_datasets import data
  import vegafusion
  from io import StringIO, BytesIO
  import json
label_desc: Use image node_id 5 for calling a global function with example usage: test_save_png() without return types