Coherence plot with multiple times and channels fails

Created by: andrew-lundgren

While using ldvw to do coherence with several channels and several start time, grouped by channel, I get the error:

``Command used to generate plot: /usr/local/ldvw/bin/gwpy-plot coherence --chan 'L1:LSC-MICH_IN1_DQ' 'L1:LSC-DARM_IN1_DQ' --start 1159691417 1159696817 --duration 20 --out '/tmp/ldvw5569505360006968315.png' --ref 'L1:LSC-DARM_IN1_DQ'

Coherence (GWpy): Status: 1 Output:

Coherence (GWpy) Stderr: Traceback (most recent call last): File "/usr/local/ldvw/bin/gwpy-plot", line 118, in result_code = prod.makePlot(args) File "/ldvwtest/home/ldvwtest/.local/lib/python2.6/site-packages/gwpy/cli/cliproduct.py", line 797, in makePlot self.gen_plot(args) File "/ldvwtest/home/ldvwtest/.local/lib/python2.6/site-packages/gwpy/cli/coherence.py", line 144, in gen_plot snd_ts.times.epoch.gps File "/ldvwtest/home/ldvwtest/.local/lib/python2.6/site-packages/astropy/units/quantity.py", line 704, in getattr attr)) AttributeError: 'Quantity' object has no 'epoch' member```