Here is an example of how to use Open Flash Chart 2 API according to ionmedia's support request #435426: how to boild charts from cck fields ?.

I was a little bit confused with his explanation but here's what I guess he wants to do:

He created a new content type called "material" and in this content type there is a field named "importance", which its value is numeric data between 1-5. He created 10 materials and assigned them their importance values. And now he wants to build a pie chart to show the summary of all materials, grouped by their importance value e.g. How many materials has importance level 1, level 2, and so on.

So I created a test site with the following setup:

1. First, I created a new content type and set its internal name to material
2. Then I created an Integer field named importance, set form element type to Select list, and set its allowed value list to be 1, 2, 3, 4, and 5
3. Next, I created 10 nodes of type material and assigned their importance to 1, 2, 1, 1, 1, 3, 4, 2, 5, 1 respectively

Now it's time to create the pie chart. I created a new Page and set input format to PHP code, then type in the following code:

<?php

// count all importance value from published (status = 1) node of type "material"
$sql = "SELECT field_importance_value AS importance, COUNT(m.nid) AS total
  FROM {content_type_material} m
  LEFT JOIN {node} n
  ON n.nid = m.nid
  WHERE n.type = 'material' AND n.status = 1
  GROUP BY field_importance_value";

$result = db_query(db_rewrite_sql($sql));

// create new chart
$chart = ofc_api_chart();

// set its title
$chart->set('title', array(
  'text' => 'Material importance',
  'style' => 'font-family: Verdana, Helvetica, Arial, serif; font-size: 20px; color: #808080;',
));

// create pie chart element
$pie = ofc_api_element('pie');

// set some effects and colors
$pie->set('alpha', 0.8);

$fade = ofc_api_element('fade');

$bounce = ofc_api_element('bounce');
$bounce->set('distance', 5);

$pie->set('animate', array(
  $fade,
  $bounce,
));

$pie->set('colours', ofc_api_color_theme('retro-spanky'));

// set label and value for each pie slice
while ($data = db_fetch_object($result)) {
  $slice = ofc_api_element();
  $slice->set('label', 'Level ' . $data->importance);
  $slice->set('value', $data->total);

  $pie->add('value', $slice);
}

// add pie element to the chart
$chart->add('element', $pie);

// render the chart
print ofc_api_render($chart);

?>

And here's the result on Flickr (Sigh, I cannot attach the image to documentation page.)

As you may already realized, the important issue here is "how do we extract the data from the database?" In this case I just use plain SQL to select the data. However, there should be a better way to do it such as using Views to build the query, etc. So please feel free to share you ideas and suggestions.

NOTE: If you want to use the data file method, just create a new module, then wrap this code snippet in a function and change the last line from print ofc_api_render($chart) to return $chart. Don't forget to add the function name in hook_ofc_api_data().