Option to include days of week on graph

For spotting trends that occur on certain days, maybe with something as simple as MTWTF. Building on top of that, if I'm only interested in finding performance data from Mondays.  If you could hover or click on 'M' and have a semi transparent column covering the entirety of each Monday shown on the graph that would likely make it easier to evaluate what's normal for that day.  It might not make sense for this to be always on, and could be an option next to the Graph and Data check boxes?