Generate graphs displaying the evolution of a Debian derivative's delta

The current code is linked from https://wiki.debian.org/Derivatives/Integration#Patches. Either compare-source-package-list could output graphs itself, or another script could take sources.{new,patches} as input, and generate graphs from it.

The goal is to visualize the evolution of a given derivative’s delta with Debian
=> detect if the situation is improving or getting out of control
=> derivatives developers can be happy and proud, or react promptly

Also, this will allow Debian contributors to evaluate how a given derivative is “nice” to Debian, FWIW :)

Blueprint: https://tails.boum.org/blueprint/better_track_and_document_our_delta_with_Debian/

Parent Task: #7607

Related issues

Blocked by #7609

Original created by @intrigeri on 7610 (Redmine)

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