The thing is that the results to render graph view is tied to the use of read_group method using the group_operator argument of the field definition that by default is sum when you don't define the group_operator. That field argument only works with aggregated functions in postgresql and percent is not an aggregated function for postgresql.

You could do it by overriding the read_group method of the graph view model to manually change the field value for the result. Like:

def read_group(self, cr, uid, domain, fields, groupby, offset=0, limit=None, context=None, orderby=False, lazy=True):
    read_group_res = super(your_model self).read_group(cr, uid, domain, fields, groupby, offset=offset, limit=limit, context=context, orderby=orderby, lazy=lazy)
    for res in read_group_res:
        domain_filter = res.get('__domain', [])
        line_ids = self.search(cr, uid, domain_filter, context=context)
        if line_ids:
            res_id = self.browse(cr, uid, line_ids[0], context=context)
            res['results'] = res_id.results
    return read_group_re

As the docstring of the read_group methods states:

:return: list of dictionaries(one dictionary for each record) containing:

* the values of fields grouped by the fields in ``groupby`` argument

* __domain: list of tuples specifying the search criteria

* __context: dictionary with argument like ``groupby``

Use this example to build your own solution based on your needs

Ibra thanks very much for your answer but the problem is that i need see the porcentage related whit all my models in a graph view, for example if i have 5 employees, 3 male and 2 female when i filter for sex in my graph view for hr_employee model i wan to see the result of all my models in porcent, not the quantity like by default odoo work. 

Hello Oscar,

you can use a widget called progressbar for this :

<field name="your_field" widget="progressbar"/>

to show the percentages, your field should be computed.
example :  add the percentage of taken seats

you .py :

seat = fields.Integer(string="Number of seats")

attendees_ids = fields.Many2many('res.partner', string="Attendees")

taken_seats = fields.Float(string="Taken seats", compute='_taken_seats')

then you add the decorator depends because taken seats depends on both seats and attendees

@api.depends('seat', 'attendees_ids')

def _taken_seats(self):

    for r in self:

        if not r.seat:

            r.taken_seats = 0.0

   else:

        r.taken_seats = 100.0*len(r.attendees_ids)/r.seat

your .xml :

<field name= "taken_seats" widget="progressbar"/>

this is a simple example on how to dislay percentage. Try to work on your own example with the same logic and tell us if it works.

Best regards.