Still the custom field is not showing in the groupby dropdown..

what is the type of field and your view is loaded in odoo properly?

It's a Datetime field.
What do you mean with view loaded in view?

You have done like this ?

< record id="hr_timesheet_line_my_timesheet_search_ext" model="ir.ui.view">
< field name="name">view.search.my.timesheet.menu.ext< /field>
< field name="model">account.analytic.line< /field>
< field name="inherit_id" ref="hr_timesheet.hr_timesheet_line_my_timesheet_search"/>
< field name="arch" type="xml">
< xpath expr="//filter[@name='filter_name']" position="after" >
< filter name="group_by_custom_field" string="Custom Field" icon="terp-partner" context="{'group_by':'custom_field'}"/>
</xpath>
< /field>
< /record>

Yes, that's exactly what I did