cli: example fails on 58a68a

On the current HEAD of the GWpy repository, the CLI example cli-spg-01.png fails to build as follows:

$ make ../build/sphinx/cli_examples/cli-spg-01.png
cli-spg-01
Traceback (most recent call last):
  File "../bin/gwpy-plot", line 118, in <module>
    result_code = prod.makePlot(args)
  File "/home/duncan.macleod/.local/lib/python2.6/site-packages/gwpy-0.1b1.dev17-py2.6.egg/gwpy/cli/cliproduct.py", line 797, in makePlot
    self.gen_plot(args)
  File "/home/duncan.macleod/.local/lib/python2.6/site-packages/gwpy-0.1b1.dev17-py2.6.egg/gwpy/cli/spectrogram.py", line 94, in gen_plot
    snr_nfft = self.dur / (secpfft * stride)
NameError: global name 'stride' is not defined
make: *** [../build/sphinx/cli_examples/cli-spg-01.png] Error 1

@areeda, can you take a look?