Visualizations: return mean and median per panel for histograms

[d-callan]

danica has a request to show a vertical line per panel indicating central tendency. not sure of priority or how others feel about the idea..

[d-callan]

shes also talking about boxplot margins for histos. might make more sense to show means in the boxplot w the other summary stats, rather than assume mean is a reasonable indicator of central tendency in a histo.

[asizemore]

Would boxplot margins be on by default? If yes, then could make sense to keep both median and mean in the boxplot to prevent redundancy. Alternatively, a 'show central tendency' toggle could turn on both mean and median vertical lines so that mean is never shown without median?

[d-callan]

think thats probably undecided yet. i also dont know if she means to have them only for the subset tab. if so, then i guess this becomes ryan's issue rather than ours.