I am working with a data-visualization application where users can view individual data items and aggregate values. Users can also "mark" individual data items; the semantics of marking are simply "this user flagged this as interesting", and cannot otherwise be derived from the data. A user can choose a marking color and that color itself might have informal semantics (like red=bad).
When viewing an individual datum it is easy to convey the marking state visually. Now consider an aggregate view, such as a bar, mark, or piece of text (like a number) representing a collection of data. For example, consider the reputation bar chart on your Stack Exchange user page and imagine that you had marked some of your questions and answers and perhaps moderators had marked others. Suppose you want to know which bars (daily totals) those questions factored into.
What is the best way to convey, in an aggregate view, that some of the data contributing to a particular grapheme (bar, mark, text, line, etc) is marked?
If a grapheme represents a small number of data items, you could do some sort of proportional coloring. (This might not work so well for text and lines.) But what if a bar represents 1000 data items, two of which are marked, one each in two different colors? A proportional representation would get lost; there aren't enough pixels to show that. If only one marking color were in play then some annotation like outlining could work, but how would that work with multiple colors? (Is there a practical upper bound? Two is probably visible, ten probably aren't.) Is it better to leave the grapheme alone and use some sort of call-out scheme?