Issue with PLOTTING files

Created by: Zaharid

This is a minimal reproducer for the problem in #1280.

dataset_input: {dataset: HERACOMBNCEP460}

use_cuts: internal

pdf: NNPDF31_nlo_as_0118

theoryid: 200

actions_:
    - plot_fancy

It fails with

Traceback (most recent call last):
  File "/home/zah/anaconda3/bin/validphys", line 33, in <module>
    sys.exit(load_entry_point('validphys', 'console_scripts', 'validphys')())
  File "/home/zah/nngit/nnpdf/validphys2/src/validphys/scripts/main.py", line 10, in main
    vp.main()
  File "/home/zah/nngit/nnpdf/validphys2/src/validphys/app.py", line 155, in main
    a.main()
  File "/home/zah/anaconda3/lib/python3.8/site-packages/reportengine/app.py", line 389, in main
    self.run()
  File "/home/zah/nngit/nnpdf/validphys2/src/validphys/app.py", line 151, in run
    super().run()
  File "/home/zah/anaconda3/lib/python3.8/site-packages/reportengine/app.py", line 374, in run
    rb.execute_sequential()
  File "/home/zah/anaconda3/lib/python3.8/site-packages/reportengine/resourcebuilder.py", line 166, in execute_sequential
    result = self.get_result(callspec.function,
  File "/home/zah/anaconda3/lib/python3.8/site-packages/reportengine/resourcebuilder.py", line 177, in get_result
    return function.final_action(fres, **prepare_args)
  File "/home/zah/anaconda3/lib/python3.8/site-packages/reportengine/figure.py", line 89, in savefiglist
    for i, fig in enumerate(figures):
  File "/home/zah/nngit/nnpdf/validphys2/src/validphys/dataplots.py", line 464, in plot_fancy
    yield from _plot_fancy_impl(results=one_or_more_results,
  File "/home/zah/nngit/nnpdf/validphys2/src/validphys/dataplots.py", line 323, in _plot_fancy_impl
    for samefig_vals, fig_data in figby:
  File "/home/zah/nngit/nnpdf/validphys2/src/validphys/utils.py", line 203, in sane_groupby_iter
    gb = df.groupby(by, *args,**kwargs)
  File "/home/zah/anaconda3/lib/python3.8/site-packages/pandas/core/frame.py", line 6717, in groupby
    return DataFrameGroupBy(
  File "/home/zah/anaconda3/lib/python3.8/site-packages/pandas/core/groupby/groupby.py", line 560, in __init__
    grouper, exclusions, obj = get_grouper(
  File "/home/zah/anaconda3/lib/python3.8/site-packages/pandas/core/groupby/grouper.py", line 811, in get_grouper
    raise KeyError(gpr)
KeyError: 'k2bins6'