Use tooltip to display aggregate (group) value?

cjmaloof

I've been using interactive tooltips to display chart data point values on mouseover -- I think they're great. However, I'm not sure how to make it work when the chart is displaying grouped values.

Specifically, with a date x-axis, I'm charting the mean / median / count of all data points from each day. But the tooltip only seems to allow me to display expressions, not aggregate functions. Is there a workaround to display the correct values?