use calendar.gmtime instead of time.mktime to avoid UTC/localtime issues with Flot
This is suggested by the Flot documentation on http://people.iola.dk/olau/flot/API.txt
"""pre 3.2 bw compat:organization: Logilab:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses"""# pylint: disable-msg=W0614,W0401fromwarningsimportwarnwarn('moved to cubicweb.selectors',DeprecationWarning,stacklevel=2)fromcubicweb.selectorsimport*fromcubicweb.selectorsimport_rql_condition