Modification to spectrogram.c for log frequency axis and setting lower/upper chart frequency

[Joe Desbonnet <jdesbonnet@gmail.com>]

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My modifications to spectrogram.c to plot frequency on log axis and also to
limit upper and lower plotted frequency are close to being complete.

I'm not very familiar with sox, and my C a bit rusty, so if any seasoned
sox developers have the time to review my code, that would be great.  The
log axis on its own didn't require much code change, but I found the
frequency limits needed a fair bit of new parsing code. Specifying
frequency limits (at least a lower bound) is necessary in log plots else a
good part of the screen will be wasted in frequencies 1 - 100 Hz which is
not the most useful for audio.

I've documented the changes at this blog post:
http://jdesbonnet.blogspot.ie/2014/02/sox-spectrogram-log-frequency-axis-and.html

And the code is here:
https://github.com/jdesbonnet/joe-desbonnet-blog/tree/master/projects/sox-log-spectrogram

If this code is worth contributing please advise what are the next steps
(if any). Consider this code contributed under whatever terms that are
required to have it added to the project.

Regards,

Joe Desbonnet.

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