bugfix: unable to plot single channel record

[biggates]

Problem:

Unable to plot a record with only one channel

Cause:

In plot.py: plot_signal(), plot_annotation() and plot_ecg_grids() uses axes as lists, e.g. :

# in plot.py: plot_signal():

    # Plot the signals
    if signal.ndim == 1:
        axes[0].plot(t, signal, sig_style[0], zorder=3)

Steps to reproduce:

>>> import wfdb
>>> record = wfdb.rdrecord('sample-data/100', sampto=3000)
>>> wfdb.plot_wfdb(record)
# This figure is well-plotted

# now try the same record, with only one channel:
>>> record2 = wfdb.rdrecord('sample-data/100', sampto=3000, channel_names=['V5']) 
>>> wfdb.plot_wfdb(record2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\git\wfdb-python\wfdb\plot\plot.py", line 642, in plot_wfdb
    return plot_items(signal=signal, ann_samp=ann_samp, ann_sym=ann_sym, fs=fs,
  File "C:\git\wfdb-python\wfdb\plot\plot.py", line 119, in plot_items
    plot_signal(signal, sig_len, n_sig, fs, time_units, sig_style, axes)       
  File "C:\git\wfdb-python\wfdb\plot\plot.py", line 283, in plot_signal        
    axes[ch].plot(t, signal[:,ch], sig_style[ch], zorder=3)
TypeError: 'AxesSubplot' object is not subscriptable

Fix:

Return a list manually if n_subplots == 1

[briangow]

@biggates, thanks for figuring this out before we got to it! It looks good to me.