What are some good papers that discuss Tufte's 'data density index' and 'data-to-ink ratio' in data visualisation?
There are critcism's of Tuft's principles.  Some examples:

Sometimes We Must Raise Our Voices, Stephen Few, Perceptual Edge


  Even after many years of working in the field of data visualization, which has involved a great deal of experience and study that has expanded my expertise into many areas that Tufte hasn’t specifically addressed, I have only on rare occasions discovered reasons to disagree with any of his principles. The topic that I’m addressing in this article, however, deals with one of those rare disagreements.


Minimalism in information visualization: attitudes towards maximizing the data-ink ratio

From the abstract:


  People did not like Tufte's minimalist design of bar-graphs; they seem to prefer "chartjunk" instead.