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/ Gnuplot / FAQ.txt

# $Id: FAQ.txt 294 2006-11-07 02:33:08Z parejkoj $

Frequently Asked Questions

This file is meant to answer the most frequently asked questions about the Gnuplot.py package. If you want to suggest additional questions (with or without answers!) please mail them to the Gnuplot.py users mailing list, <gnuplot-py-users@lists.sourceforge.net>.

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Q1:

When running the following script

------------------------------------------------ #! /usr/bin/python2

import Gnuplot, Gnuplot.funcutils from numpy import *

g = Gnuplot.Gnuplot() g.plot([[0,1.1], [1,5.8], [2,3.3], [3,4.2]]) ------------------------------------------------

### I get the error

gnuplot> plot `/tmp/@24463.2` ^ can't read data file "/tmp/@24463.2" line 0: (No such file or directory)

However, the same commands work fine if I type them into the interpreter! What's wrong?

A1:

The problem is that in many cases Gnuplot.py sends data to gnuplot via a temporary file. But Gnuplot.py has no way of knowing when it is safe to delete the temporary file. So it deletes it when the corresponding PlotItem object is deleted, which is typically when the next Gnuplot.plot() command is executed or when the python script ends. (Until you plot something else, the Gnuplot object keeps a reference to all of the old plot items to prevent their being garbage collected.)

To prevent this problem, there are several possibilities:

1. Switch to Unix. On that platform, Gnuplot.py now uses FIFOs (named pipes) by default to send data to gnuplot. This seems to provide a robust and somewhat elegant solution to this problem.
2. Use "inline data" instead of temporary files to communicate with gnuplot. This is already available in gnuplot for most 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. Since inline data doesn't involve temporary files, the problem goes away.
3. 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 if you are producing a graphics file you might watch for the creation of the output file and at that point assume that gnuplot is done with the temporary file. One idea is to explicitly create a PlotItem to represent the data, and keep a reference to the PlotItem for some time after the plot() command is executed; e.g.,

   data = Gnuplot.Data([[0,1.1], [1,5.8], [2,3.3], [3,4.2]]) g.plot(data) # ... do something guaranteed to last a couple seconds del data # temporary file is deleted at this moment

4. Change Gnuplot.py itself to implement two-way communication between gnuplot and Gnuplot.py. Then, for example, Gnuplot.py could delete temporary files when the next gnuplot prompt appears. This would be a lot of work but it would allow other new features such as detecting gnuplot errors, reading gnuplot fit command output back to python, etc.

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Q2:

Does Gnuplot.py work under Jython/Java?

A2:

Partly. Version 1.7 added the low-level interface gp_java.py for communicating with gnuplot using the Java libraries, and that part seems to work.

However, Gnuplot.py relies on the Python Numeric library, which is a C library. The Jython equivalent, called JNumeric <http://jnumerical.sourceforge.net/>, therefore has to be installed. However, JNumeric is still at beta level, and operation under Jython hasn't been tested much at all, so feedback is welcome!

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Q3:

[from Tate Wilson]

I am trying to set up your gnuplot/python package on my mac (osX). The readme file says I need to convert the files to mac text files. The coverter I usually use, maclink, won't handle these files. Can you give me a hint how to convert the files or where to look for help?

A3:

I don't know. But the same user later reported what worked for him:

I did have to convert the files. It may not be true for all mac Python users, but I suspect it is. I'm using a graphical Python development environment called "Python IDE for mac" which may have its own pickiness, but still, it wouldn't even recognize the files in your package as being the type it could open. I tried a few different file converters with no luck. Then I just opened all the files with BBedit, changed something so I would be prompted to save it on closing (like add and remove a letter), and closed BBedit. Then all the files were `mac` files and the Python interpreter recognised them.

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Q4:

I am using Windows and I get an error like

> Traceback (most recent call last): > [...] > File "C:\Python23\Lib\site-packages\Gnuplot\gp_win32.py", > line 125, in __call__ > self.write(s + `\n`) > IOError: [Errno 22] Invalid argument

What is the problem?

A4:

This is apparently the error that results when Gnuplot.py cannot start the pgnuplot.exe executable under Windows. It could be that gnuplot (the plotting program, as opposed to Gnuplot.py) is not installed, or that it is not in your PATH. If your pgnuplot.exe executable is named differently, or you do not want to add its directory to your PATH, you can change gnuplot_command in gp_win32.py to indicate the precise path to the executable.

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