Correct docstring for plot.py

[johndgiese]

When running the latest version of the code, the example didn't run without adding these two arguments.

[tompollard]

The existing code fails with the following error:

import wfdb

record = wfdb.rdrecord('sample-data/100', sampto=3000)
ann = wfdb.rdann('sample-data/100', 'atr', sampto=3000)

wfdb.plot_items(signal=record.p_signal,
                ann_samp=[ann.sample, ann.sample],
                title='MIT-BIH Record 100', time_units='seconds',
                figsize=(10,4), ecg_grids='all')

Traceback (most recent call last):
  File "/Users/tompollard/projects/wfdb-python/wfdb/plot/plot.py", line 488, in _plot_signal
    t = tarrays[ch_len, ch_freq]
KeyError: (3000, None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tompollard/projects/wfdb-python/wfdb/plot/plot.py", line 261, in plot_items
    _plot_signal(
  File "/Users/tompollard/projects/wfdb-python/wfdb/plot/plot.py", line 495, in _plot_signal
    "minutes": ch_freq * 60,
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

Updating the arguments as suggested fixes this issue:

import wfdb

record = wfdb.rdrecord('sample-data/100', sampto=3000)
ann = wfdb.rdann('sample-data/100', 'atr', sampto=3000)

wfdb.plot_items(signal=record.p_signal,
                ann_samp=[ann.sample, ann.sample],
                fs=record.fs, sig_units=record.units,
                title='MIT-BIH Record 100', time_units='seconds',
                figsize=(10,4), ecg_grids='all')

Presumably the behaviour of plot_items changed at some point. It feels like this shouldn't be failing just because these arguments aren't provided (e.g. if units are unknown, just display "Unknown").

[tompollard]

Thanks @johndgiese!