raw.plot does not work with show_first_samp

Description of the problem

Plotting raw data with show_first_samp=True gives strange results.

Steps to reproduce

import numpy as np
import mne

sample_data_folder = mne.datasets.sample.data_path()
sample_data_raw_file = (sample_data_folder / 'MEG' / 'sample' /
                        'sample_audvis_filt-0-40_raw.fif')
raw = mne.io.read_raw_fif(sample_data_raw_file)

events = mne.make_fixed_length_events(raw)
raw.plot(events=events, event_color='r', show_first_samp=True)

Link to data

No response

Expected results

We would expect the data to be shown but it is not. It seems to suggest some indexing bug.

Actual results

Additional information

Platform:         Linux-3.10.0-1160.76.1.el7.x86_64-x86_64-with-glibc2.10
Python:           3.8.6 | packaged by conda-forge | (default, Oct  7 2020, 19:08:05)  [GCC 7.5.0]
Executable:       /autofs/space/meghnn_001/users/mjas/anaconda3/envs/mne/bin/python3.8
CPU:              x86_64: 64 cores
Memory:           125.4 GB

Exception ignored on calling ctypes callback function: <function _ThreadpoolInfo._find_modules_with_dl_iterate_phdr.<locals>.match_module_callback at 0x7f80b4d9b040>
Traceback (most recent call last):
  File "/autofs/space/meghnn_001/users/mjas/anaconda3/envs/mne/lib/python3.8/site-packages/threadpoolctl.py", line 400, in match_module_callback
    self._make_module_from_path(filepath)
  File "/autofs/space/meghnn_001/users/mjas/anaconda3/envs/mne/lib/python3.8/site-packages/threadpoolctl.py", line 515, in _make_module_from_path
    module = module_class(filepath, prefix, user_api, internal_api)
  File "/autofs/space/meghnn_001/users/mjas/anaconda3/envs/mne/lib/python3.8/site-packages/threadpoolctl.py", line 606, in __init__
    self.version = self.get_version()
  File "/autofs/space/meghnn_001/users/mjas/anaconda3/envs/mne/lib/python3.8/site-packages/threadpoolctl.py", line 646, in get_version
    config = get_config().split()
AttributeError: 'NoneType' object has no attribute 'split'
mne:              1.4.dev0
numpy:            1.23.0 {OpenBLAS 0.3.12 with 1 thread}
scipy:            1.5.3
matplotlib:       3.3.3 {backend=Qt5Agg}

sklearn:          0.23.2
numba:            Not found
nibabel:          3.2.1
nilearn:          0.7.0
dipy:             1.3.0
openmeeg:         Not found
cupy:             Not found
pandas:           1.1.5
pyvista:          0.36.1 {OpenGL 4.5.0 NVIDIA 455.45.01 via Quadro P5000/PCIe/SSE2}
pyvistaqt:        0.9.0
ipyvtklink:       Not found
vtk:              9.0.1
qtpy:             2.0.1 {PyQt5=5.12.9}
ipympl:           Not found
pyqtgraph:        0.13.1
pooch:            v1.5.2

mne_bids:         Not found
mne_nirs:         Not found
mne_features:     Not found
mne_qt_browser:   0.4.0
mne_connectivity: Not found
mne_icalabel:     Not found

Run the supplied Python reproduction using MNE's raw.plot with events and show_first_samp=True, then trace the raw.plot entry point and its event or sample-index handling. The fix is done when the raw data and event markers are displayed correctly with show_first_samp=True, without regressing the normal plotting case.