One of my favorite examples is the semi-transparent bubble from SurveyGizmo:
But the format that you are already using for calling out data points on graphs is great too. Basically just make it so when a user hovers over the bar graph they get the number, as they do now, but when they hover over the metric name they get a glossary definition of the metric.
A couple concessions: you could start with "Custom Metric" for any metrics that are non-standard. Eventually, it would be great to have those metrics call-outs be their equation.
Not sure what to do about filtering, but it would be pretty cool to be able to call out filtering in the same way.