Follow these steps to add the cohort view for the project.task model:

1. Define a cohort view:

<record id="view_project_tasks_graph" model="ir.ui.view">
    <field name="name">project task cohort</field>
    <field name="model">project.task</field>
    <field name="arch" type="xml">
        <cohort date_start="date_start"
            date_stop="date_deadline"
            interval="month"
            string="Task Cohort" />
    </field>
</record>

2. Add menus and actions using this view. This is left as an exercise for the reader.