[mod_python] does a mod_python handler exist to generate charts/graphs? (png images)

David,
   Thanks for the input.  I'll add chaco and gnuplot.py to the list.  So 
far I have (in no order):

reportlab
pygdchart
chaco
gnuplot

I'll have to start investigating each of them.  chaco and gnuplot appear 
to have massive options and functionality.


In response to your earlier suggestions:
"""
This kind of thing is pretty simple, you can either create a 
StringIO.StringIO object (standard Python) which is a pseudo-file that 
you can get the contents out of using a string, or you could try passing 
the request object (at the appropriate time) and see if it will work 
writing to that.
Another option (assuming the project is open source) is to alter the 
program to support returning the data in a string.
I got the reportlab guys to do this - their project is mainly focussed 
on Reports and PDF, but it also has charting capabilities I haven't 
investigated.
"""

I tried to use the StringIO object from both StringIO and cStringIO.  The 
library didn't like
that at all.  I traced some of the pygdchart code.  It drops down into C 
and calls the following code (from _gdchartc.c)
( I stripped out some unnecessary lines):
PyObject *pfile, *labels, *data, *combodata;
FILE *fp;
...
if (!PyArg_ParseTuple(args, "iiO!iiOiOO", &width, &height, &PyFile_Type, 
&pfile, 
...
fp = PyFile_AsFile(pfile);

I so new to python it hurts, but I'm guessing that at this point in a C 
program, pfile has to be an
actual pointer to a file and not a file-like interface such as StringIO? 
Any experience with this?

As for having the author change the interface, I've already emailed them. 
Later on I found
the cStringIO functionality listed in a TODO file for the pygdchart 
library.  Guess I'm not the first...
I don't know how possible it would be to use cStringIO, since the FILE 
pointer is passed through a mess of C libraries

gdchart -> gd -> libjpeg

The functions all the way down to libjpeg are wanting a pointer to a FILE 
structure.   My other thought was
looking for a way to imitate the FILE pointer in memory and modifying the 
entry C code to return back the
in memory data as a binary stream.  I'll have to look into that as well. 
My C is pretty weak as well.  I'm
so thankful there's Google.

As a workaround, here's what I did (using os.tmpfile):

from mod_python import apache
from mod_python import util
import gdchart
import os

def handler(req):
        req.content_type = "image/png"

        chart = gdchart.Line3D()
        chart.width = 250
        chart.height = 250
        chart.xtitle = "xtitle goes here"
        chart.ytitle = "Percentage"
        chart.title = "Example Graph"
        chart.ext_color = [ "white", "yellow", "red", "blue", "green"]
        chart.setData([20, 100, 80, 30, 50],[10,150,20,25,65])
        chart.setLabels(["Mon", "Tue", "Wed", "Thu", "Fri"])
        chart.setLabels(["Mon", "Tue", "Wed", "Thu", "Fri"])

        f = os.tmpfile()
        chart.draw(f)
        f.seek(0)
        req.write(f.read())
        f.close()

        return apache.OK

It outputs a graph for me.    Here's another question that may be 
better suited for comp.lang.python, but do you know if
os.tmpfile() uses an in-memory representation of a file
or does it use a physical file that just remains anonymous?  I'm 
concerned about the wasteful loss of speed having to write to
disk and read it back.

Well, enough rambling.  Thanks for your time,
Perry Tew












David Fraser <davidf at sjsoft.com>
09/18/2003 04:45 PM

 
        To:     Perry Tew/CV/Novartis at PH
        cc:     mod_python at modpython.org
        Subject:        Re: [mod_python]        does a mod_python handler exist to generate 
charts/graphs? (png     images)


perry.tew at cibavision.novartis.com wrote:

>>perry.tew at cibavision.novartis.com wrote:
>>
>>>Hello everyone,
>>>  I was about the go down the road of developing a handler to generate 
>>>charts,
>>>similar to the mod_perl Apache::GD::Graph.  I wanted to make sure 
>>>something doesn't exist
>>>before I reinvent the wheel.  Does anyone know of such a thing yet?  If 

>>>so, would you please
>>>provide a url?
>>>
>>>Thanks,
>>>Perry Tew
>>
>> There are several Python graphing packages which you could use towards
>>
>>this end, the question is how nicely will they plug into a web 
>>environment.
>>I am not aware of any that are set up to do this using mod_python.
>>I'm also busy looking at the problem, for an application we are 
>>currently converting to be web based, so discuss plans / things you find 

>>here and I will to and hopefully we can come up with a nice solution...
>>
>>Thanks,
>>David
>>
>
>David,
>  Thanks for the information.  I was looking at using pygdchart2 from 
>nullcube.  I was pleased with the interface they've written.  Have you 
any 
>experience with them?  I figured it shouldn't be technically difficult to 

>create an interface of parameters that mapped to chart settings.
> 
>
I haven't had, but will look at it, do you have a URL?

>The only concern I have so far is the method of outputting the chart once 

>rendered.  The examples use the draw method:
>"""
> chart.draw(filespec)
>Draw the graph to the specified file. If the filespec argument is a 
>string, it will be treated as the path to a file. Otherwise, filespec is 
>assumed to be a Python file object.
>"""
>
>I'm new to mod_python, so I'm not sure how to pass the output stream of 
>the request object to the draw() method.  If you know, I'm be grateful. 
If 
>not, I'll work at it.  I haven't really dug yet.
> 
>
This kind of thing is pretty simple, you can either create a 
StringIO.StringIO object (standard Python) which is a pseudo-file that 
you can get the contents out of using a string, or you could try passing 
the request object (at the appropriate time) and see if it will work 
writing to that.
Another option (assuming the project is open source) is to alter the 
program to support returning the data in a string.
I got the reportlab guys to do this - their project is mainly focussed 
on Reports and PDF, but it also has charting capabilities I haven't 
investigated.
The other charting toolkit I am aware of is Chaco.

I think it would be good to investigate all of them and at least draw up 
a list of their strengths/weaknesses for use in a web server context, 
that might be helpful to other users. Would be good to have a list of 
requirements to evaluate these packages by e.g. how big the package is, 
able to produce different image formats, etc.

>btw,   I saw your post earlier about your wiki.  While cruising your site 

>I found a MLM I was in need for.  Thanks!
> 
>
Sure, but I think it was somebody else replying to my message, I don't 
have a wiki...

>Perry Tew
> 
>
David

PS hope you don't mind me rearranging your post, bottom-posting makes 
the flow easier to see.