Easily plot realtime data (via Perl and GnuPlot) | Hacker News Reader

Easily plot realtime data (via Perl and GnuPlot)

users.softlab.ntua.gr

39 pointsttsiodras14 comments

Line 5 just made me spill my coffee (or I need to drink some more). Since STDOUT is the default filehandle, there is no need to muck around with that anachronism. A simple $| = 1 would suffice. This idiom was once useful for setting autoflush for a filehandle other than STDOUT, but this is not the case here. For a lengthier discussion on this, including the apologies of Randal Schwartz for coming up with it (when admittedly other facilities were not available for the very specific use case it addresses) see [0].

[0] http://stackoverflow.com/questions/196754/what-does-selectse...

Back in 2008 (when this was written) "perldoc -q flush" suggested exactly that gem :-) I admit I've more or less abandoned Perl since then (scripting in Python for everything).

A nicer way to do it that's not as obtuse, since 5.6 is to do

  use IO::Handle;
  *STDOUT->autoflush(1);

This ends up far clearer than either example, and the idiom even extends easily to all file handles, lexical or otherwise.

EDIT: That being said,

  system("sleep 0.02") == 0 or last

is even worse. There is no need to go out to a shell or even an external program for that.

  use Time::HiRes;
  ...
  sleep(0.02);
  ...

Not only will that work on far more systems, it will use less resources and is available to all perl's since 5.7.3 (development release from March 2002, first in stable release from July 2002).

Thanks for this. Note that the "sleep" is not part of the plotting script - it's just an example that generates data for plotting. I updated the page with your suggestion.

just a correction, you need to import the sleep function from Time::HiRes, otherwise it will use the default sleep function and sleep 0 seconds instead.

    use Time::HiRes qw/sleep/;

Perl/shell utilities to drive gnuplot come over and over on HN and Reddit. Gnuplot is _slow_. There's a better (and easier) way to deal with realtime data:

http://www.thregr.org/~wavexx/software/trend/

It's already been used for all kind of stuff (https://projects.ardrone.org/boards/1/topics/show/491, ECG, etc).

Very cool! I use gnuplot and a simple Ruby script [1] to plot my ab results. I may take a crack at adapting this technique to a Ruby script so I can see my results in real time.

1 - https://github.com/bradland/apache-bench-grapher

Got a better one for you: https://github.com/dkogan/feedgnuplot

As the script from the OP, this is a tool to plot STDIN. This one has been refined far more, so it's trivial to use and very powerful.

"This program is originally based on the driveGnuPlots.pl script from Thanassis Tsiodras. It is available from his site at http://users.softlab.ece.ntua.gr/~ttsiod/gnuplotStreaming.ht...

:-)

How is that simpler than, e.g., Python's matplotlib?

matplotlib is a library; this is a commandline tool. The Perl part is largely irrelevant.

What is simpler depends on which method you know. I'd also use matplotlib in this case, if you already know Python it's great for some quick plotting.

Have you tried using tcl/blt?

Perl is like a corpse of a beautiful woman. Tempting, but still so very wrong.

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