This is a simple quick'n'dirty chart generator for erlang systems. The erlang side consists of a little process that takes statistics every soandso many milliseconds, and writes them to a file in JSON format using mochijson2.

The perl side uses JSON (pure perl) or JSON::XS (C backend, faster by the order of magnitudes) to parse the json, and Chart::Strip to output nice-looking charts in png or jpg format.

SETUP:

THE ERLANG PART:
just let your supervisor start the statistics process like you would start any other process (i.e. using statistics:start_link()).
mochijson2.erl needs to be compiled and in your codepath.

THE PERL PART:
first install the following prerequisites:
- JSON
- Chart::Strip

you can do so using your package manager, cpan or cpanp.
Using the local package manager usually preferable. For instance on ubuntu you would type:
$ sudo apt-get install libjson-perl libchart-strip-perl
or, to get the faster json-xs backend:
$ sudo apt-get install libjson-perl libjson-xs-perl libchart-strip-perl

Visa cpan:

$ cpan
CPAN Terminal> install JSON
... lots of output ... 
All OK.
CPAN Terminal> install Chart::Strip
... lots of output ...
All OK.

To verify whether a perl library was successfully installed you can use the following command:
perl -MLibraryname -e 'print "All OK!\n"'
so for example:
perl -MJSON -e 'print "All OK!\n"'

CONFIGURING THE PERL PART
the perl part is configured through a json config file, located in perl/config.jsn.


ADDING GENERIC CHARTS
This version of erlang-statistics also supports adding arbitrary data to the json packet, so that you can use it to chart whatever you want, without
having to actually do any programming in perl.

the default configuration plots some fake data, you can disable it if you don't need it.

To do this, you need to add your payload to the json packet like this:

"generics":[
    {
      "key":1,
      "anotherkey":2
    }
  ]

To do this, on the erlang side, edit the function get_additional_statistics/0 like so:

get_additional_statistics() ->
  [
    {<<"generic">>, {struct, [
        {<<"key">>, int()},
        {<<"anotherkey">>, int()}
    ]}}
  ].
the key has to be an unique identifier.

Then you have to add to the configuration:

{
            "class":"Generic",
            "filename":"stuff.png",
            "style_options":{
		"type":"cumulative" // this can be "cumulative" or "normal". Cumulative assumes your data is growing all the time, "normal" assumes its not. usually, you want cumulative.
		"data":[            // field that describes where to pull the data from.
		  {
		    "label":"Some stuff I want to measure", // label that appears on the graph
		    "key":"key"                             // the key to get the data from.
		  },
		  {
		    "label":"Some more stuff I want to measure",
                    "key":"anotherkey"
		  }
		],
                "y_label":"Label on the y axis"    
            }
        }
This will cause Stat::Generic to render a chart from custom userdata with the given options. You can repeatedly add this to the configuration to render multiple charts,
you can pull as many keys into one chart as you wish, and you can put as much additional information into the json hash as you like.

Additional note on how to fetch the data:
usually, you want to fetch the data in each tick. Cumulative data usually gives the most precise results, e.g. don't set your data to zero after each tick, but
let it increase. Sending data to the statistics process instead of fetching it from it, usually leads to less precise results.
(it is, however, of course perfectly fine to use the statistic process' state to accumulate data, and then write it out on each tick.)