def extent(self, args, time, cffile=None, cntrtype="c"):
"""Plot ice extent.
args: further arguments
time: time slice
cntrtype: contourtype, c for normal contour, a for annotated"""
if cffile == None:
cffile = self.file
extent = CFvariable(cffile, "thk").getGMTgrid(time)
extent.data = extent.data - 0.1 + cffile.time(time)
PyGMT.AreaXY.contour(self, extent, [cffile.time(time)], args, cntrtype)
