add some data

def add_data(self, t, msg, vars):
        '''add some data'''
        mtype = msg.get_type()
        for i in range(0, len(self.fields)):
            if mtype not in self.field_types[i]:
                continue
            f = self.fields[i]
            simple = self.simple_field[i]
            if simple is not None:
                v = getattr(vars[simple[0]], simple[1])
            else:
                v = mavutil.evaluate_expression(f, vars)
            if v is None:
                continue
            if self.xaxis is None:
                xv = t
            else:
                xv = mavutil.evaluate_expression(self.xaxis, vars)
                if xv is None:
                    continue
            try:
                v_f = float(v)
                xv_f = float(xv)
            except Exception:
                continue
            self.y[i].append(v_f)
            self.x[i].append(xv_f)