I have been exploring JSTOR's Understanding Shakespeare API. A few years ago, JSTOR Labs, in collaboration with the Folger Library, ran a Regular Expressions search of every line from the Folger's Digital Texts against their entire OCR'd database of publications. The result is a sort of index to Shakespeare citations in JSTOR.
As I work to write up some comprehensive results from my work with this data, you can find preliminary visualizations below.
This page gives the full text of each of Shakespeare's 38 plays in columns. The plays are sorted chronologically from left to right. The text of each play is color-coded on a scale from yellow to purple, scaled according to the total number of citations to each play. (Hamlet is black because it so far outstrips the other plays that including it on the color scale makes the separation among the other plays impossible to discern.) Within each play, the transparency of each line is scaled according to the relative citational frequency of the line compared to the rest of teh play. So, the most cited line is 0% transparent and uncited lines are 100% transparent (i.e., invisible).