Allow querying of multiple repositories simultaneously

It would be awesome if one could query multiple repositories simultaneously and download data for several repos in the same CSV file. Here's how it would ideally work, according to me:
- You provide repository names separated by commas (e.g. rubinius/rubinius, klangner/matrobot etc.)
- A graph is provided where you can switch on and off various plots
- A CSV containing the data for all the repos can be downloaded

No files, tests, or entry points are named. Start by locating the existing repository query, graph, and CSV download flows; done means comma-separated repositories can be queried together, their plots can be toggled, and one CSV contains data for all selected repositories.

Rédigé par le modèle d'indexation à partir du texte de l'issue.