Once you have the Matplotlib wheel, pip is still trying to reach out to the internet to get the wheels for our dependencies.
https://stackoverflow.com/questions/9698557/how-to-use-pip-on-windows-behind-an-authenticating-proxy maybe helpful, however if you are behind an authenticated proxy I suspect that you are on a corporate network. I suggest re-directing this question to your local helpdesk to sort out how to configure this and still be complaint with your local policies.
The other option is to download the wheels for each of our dependencies (and their dependencies …) by hand.