Hello Alain 

Odoo sub analytic account is little confusing. here is a simple explanation of how you can use it. 

Create Analytic Plan e.g. Department 
Create Sub Plan
- Marketing 
- Human Resource 

Create Analytic Account and assign them to the sub plan.

e.g. 
Digital Marketing -> Marketing 
Stationary -> Human Resource 

Now create a Vendor bill and assign the analytic distribution as follow 

Vendor Bill line 
Transportation - Cost 1000
Analytic Distribution 
Digital Marketing - 50%
Stationary - 50% 

To view the report. Go to Analytic Reporting 
From left side click on the Plus icon next to the total and Select The department. It should ask to select one of the subplan. Select one of them and it should break down the cost by sub plan. 

I hope this helps