function ModifDateDeb (date_deb, date_fin, nb)
        {
                if (date_deb == date_fin || date_deb < date_fin) return;

                for (i=0; i < nb; ++i)
                        if (document.form_graph.date_fin.options[i].value == date_fin)
                        {
                                if (i == 0) iddeb = i + 'deb';
                                else iddeb = i - 1 + 'deb';
                                break;
                        }
                document.getElementById(iddeb).selected='selected';
                return;
        }
        function ModifDateFin (date_deb, date_fin, nb)
        {
                if (date_deb == date_fin || date_deb < date_fin) return;

                for (i=0; i < nb; ++i)
                        if (document.form_graph.date_deb.options[i].value == date_deb)
                        {
                                if (i == nb - 1) idfin = i + 'fin';
                                else idfin = i + 1 + 'fin';
                                break;
                        }

                document.getElementById(idfin).selected='selected';
                return;
        }