Installing pygal under Ubuntu 18.04

[ybeco]

I don't understand why there is this message, while it asks me root password...

But it says that "cffi" is OK in python 3.6

Output:
yonnel@yonnel-Z97X-Gaming-7:~$ sudo python3 -m pip install cffi
[sudo] Mot de passe de yonnel : 
The directory '/home/yonnel/.cache/pip/http' or its parent directory is not owned by the current user and the cache has been disabled. Please check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
The directory '/home/yonnel/.cache/pip' or its parent directory is not owned by the current user and caching wheels has been disabled. check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
Requirement already satisfied: cffi in ./.local/lib/python3.6/site-packages
Requirement already satisfied: pycparser in ./.local/lib/python3.6/site-packages (from cffi)
yonnel@yonnel-Z97X-Gaming-7:~$ 

When I run "foobarbaz.py" with python 3.7 IDLE, I still have the same error message...

But when I run "foobarbaz.py" with python 3.6 in Bash terminal, It's working ! It dont shaw anything, but it makes a "foo.png" file with the chart

--> Is it posible to change the version of Python that the IDLE is runing?