Fix: Fix graph for product distribution.

htdocs/comm/propal/class/propal.class.php

htdocs/comm/propal/class/propalestats.class.php

htdocs/commande/class/commandestats.class.php

htdocs/compta/facture/class/facturestats.class.php

htdocs/core/boxes/box_graph_invoices_permonth.php

htdocs/core/boxes/box_graph_orders_permonth.php

htdocs/core/boxes/box_graph_product_distribution.php

htdocs/core/boxes/box_graph_propales_permonth.php

htdocs/core/class/stats.class.php