[Gnuplot-py-users] Re: gnuplot-py problems

[Michael H. <hag...@jp...>]

Dear Jonathan,

Jonathan Ricketson writes:
> Hey Michael, the interface that you have written for gnuplot is
> really good.  But I am having a problem with using it from a web
> browser.
> 
> The funny thing is that it works fine from the command line but when
> I try to run the script from a browser it has this problem:
> 
> gnuplot> set title "Some Title"
> gnuplot> set term png
> gnuplot> set output "/var/www/sample.png"
> gnuplot> set data style linespoints
> gnuplot> plot '/tmp/@18326.0' notitle
> 
> gnuplot> plot '/tmp/@18326.0' notitle
>               ^
>          can't read data file "/tmp/@18326.0"
>          line 0: (No such file or directory)
> 
> the problem seems to be that the tmp file is deleted too quickly, or
> doesn't exist yet... Because from the command line I can replicate
> the problem by the following code:
> 
>   g.plot(data)
>   raw_input('Please press return to continue...\n')
> 
> If I keep the raw input line, then it works, otherwise if I take it
> out, then it has the same error as above.
> 
> If you have any idea what the problem might be I would really
> appreciate your help.
> 
> Jonathan

Your diagnosis is exactly correct.  Communication with gnuplot is via
temporary files by default, and the temporary files are deleted as
soon as the "Gnuplot.Data" object is garbage collected, which in your
example probably occurs when the script ends or when you plot
something else.  (Until you plot something else, the Gnuplot object
keeps a reference to all of the old plot items to prevent they're
being garbage collected.)  There is no way for Gnuplot.py to know when
gnuplot is done reading the temporary file, so there is no general way
for it to know when it is actually safe to delete the file.

Possible solutions are:

1. Use "inline data" instead of temporary files to communicate with
   gnuplot.  This is already available in gnuplot for many plotting
   modes if you specify the "inline=1" option to the constructor of
   the Data object, or if you set GnuplotOpts.prefer_inline_data=1.

2. Introduce a delay between the time you plot and the time you allow
   the Data object to be deleted.  You could just use time.sleep(), or
   in your example you might watch for the creation of the
   /var/www/sample.png file and at that point assume that gnuplot is
   done with the temporary file.

3. Improve Gnuplot.py for all of us by adding an option to use named
   pipes instead of temporary files.  I think it should be possible
   for Gnuplot.py to know when gnuplot has finished reading the data
   from the named pipe so that there is no more ambiguity.  Note that
   it will take some effort to make such a solution portable across
   operating systems, which is the main reason that I haven't
   undertaken it already.

I have directed this reply to the gnuplot-py-users mailing list.
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Yours,
Michael

--
Michael Haggerty
hag...@jp...