Define your own period

thanks for your request,

unfortunately this is a little complicated as piwik either wants the period to be a string like "day", "week", "month", "year" or it wants to have a specific date "range". That means that a string like "last 2 days" must be converted into a date range. (See piwik API documentation for reference).

To make a good implementation of this we need to find an elegant way to transform thresholds into date ranges

yes that would be easily possible, but that would not make possible to define it like "last two days" (unless you edit the fields config every day). I already thought about just making this range setting available but I couldn't think of any usecase worth the effort. (why does somebody just want static node statistics from a date range?)

yes but custom ranges won't help in your example

Period: custom
Range: 22.12.2011 - 24.12.2011
now you have to change the date every day manually, otherwise it will show statistics from 22.12.2011 - 24.12.2011

What you need is a custom dynamic range like
"last two days"
and this must automatically be translated into
[current date - 2days] until [current date]
before submitting it to piwik api

this is the problem I meant with - how to solve this in an elegant way

What do you think about next solution?

First, I propose to add new parameters to widget settings ('range-2' and 'range-3' in this example):

function piwik_stats_field_settings_form($field, $instance, $has_data) {
  $settings = $field['settings'];
  // Add field settings for statistic period.
  $form['period'] = array(
    '#type' => 'select',
    '#title' => t('Period'),
    '#default_value' => $settings['period'],
    '#options' => array(
      'day' => t('Day'),
      'range-2' => t('2 days'),
      'range-3' => t('3 days'),
      'week' => t('Week'),
      'month' => t('Month'),
      'year' => t('Year'),
    ),
    '#required' => TRUE,
    '#description' => t('The period of the requested statistics.'),
  );
  return $form;
}

Then add next code to the function piwik_stats_api_request:

if (strstr($period, 'range-')) {
  $range = str_replace('range-', '', $period);
  $date = date('Y-m-d', time() - 86400 * $period) . ',' . date('Y-m-d');

  $period = 'range';
}

In result this function would look like this:

function piwik_stats_api_request($piwik_url, $token_auth, $method, $site_id, $period = 'year', $date = 'now', $format = 'xml') {
  if (strstr($period, 'range-')) {
    $range = str_replace('range-', '', $period);
    $date = date('Y-m-d', time() - 86400 * $period) . ',' . date('Y-m-d');

    $period = 'range';
  }

  // Send off the API request.
  return drupal_http_request(
    url($piwik_url,
      array(
        'query' => array(
          'module' => 'API',
          'method' => $method,
          'idSite' => $site_id,
          'period' => $period,
          'date' => $date,
          'format' => $format,
          'token_auth' => $token_auth,
          'expanded' => TRUE,
        ),
      )
    )
  );
}

would be a way, but this means all ranges are hardcoded and only the ranges available in the select box can be chosen,

I'd rather like to have a "Range" item in the period select and if you select range another textfield apears where you can enter a range like "+2 days", "+6 hours", etc.
This string can then be translated into a range of the "current_time" until "strtotime(range, current_time)".

$date = date('Y-m-d', strtotime($range)) . ',' . date('Y-m-d');

$form['period'] = array(
    '#type' => 'select',
    '#title' => t('Period'),
    '#default_value' => $settings['period'],
    '#options' => array(
      'day' => t('Day'),
      'week' => t('Week'),
      'month' => t('Month'),
      'year' => t('Year'),
      'range' => t('Custom range'),
    ),
  );

marked #2058935: Statistics field settings 'Period' as duplicate of this feature request.

this is my solution:

//$X number of days to sum the reports up.
$period = 'range';
$date = 'previous' . $X;

See http://piwik.org/docs/analytics-api/reference/ for explanation of lastX and previousX