[Fixed]-How to store nested lists as Django model field (or should I keep data as flat files)?

Since you read/write you should store data in your database. You can create a model which corresponds to your csv. A nested list can be stored as a simple string (or alternatively, if you need to query the list directly you can create another model which will hold the values – but that’s rather inefficient). As for importing/outputting csv with models, you can do that with a script of your own or use a library like django-adaptors. Oh, and as far as I am aware of D3.js accepts JSON which is more flexible and better supported in Django.