Is that Citi Bike station dead? When were bikes shifted from that station? When did station z go to 0 bikes or 0 docks?
This tracks the data in RRDtool.
(the largest station, called 8 Ave & W 31 St, just west of Penn Station on the evening of 6/15/2013)
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Install RRDtool, perhaps.
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Set WD in pollup to something handy.
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Run pollup once by hand ignoring the error.
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grep -v Planned grab | cut -d ' ' -f 1 | cut -c6-9 > id -
Edit file 'id' to remove any stations you do not care about. (Runs fine with all stations on any contemporary machine.) The left columns in 'grab' are the only key.
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Run massrrd.
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Have pollup run every minute from cron.
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Wait a few minutes.
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Run draw.
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Wait more.
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Go to 9.
If you recreate 'id' and new stations appear you'll see something like — ERROR: opening 's241.rrd': No such file or directory — when you run draw. Generate its RRD file by hand using the line from massrrd.