Hi Pascal,

The default type of the graph is a pie chart

Bt in the POS , they changed the graph type as "pivot"  or  "bar",

For example:

<graph string="Point of Sale Analysis" type="pivot"> .

So you can change this to:

<graph string="Point of Sale Analysis" type="pie">

if type attribute doesn't work for you try to add this context in your window action

<field name="context">{ 'graph_mode': 'pie' }</field>

this worked for me