diff --git a/htdocs/core/class/stats.class.php b/htdocs/core/class/stats.class.php
index ae9a8e56e58..a31c9fc3349 100644
--- a/htdocs/core/class/stats.class.php
+++ b/htdocs/core/class/stats.class.php
@@ -33,6 +33,13 @@ abstract class Stats
 	protected $lastfetchdate = array(); // Dates of cache file read by methods
 	public $cachefilesuffix = ''; // Suffix to add to name of cache file (to avoid file name conflicts)
 
+	/**
+	 *  @param	int		$year 			number
+	 * 	@param	int 	$format 		0=Label of abscissa is a translated text, 1=Label of abscissa is month number, 2=Label of abscissa is first letter of month
+	 * 	@return int						value
+	 */
+	protected abstract function getNbByMonth($year, $format = 0);
+
 	/**
 	 * Return nb of elements by month for several years
 	 *