I was surprised that I could not create a bar chart that would sum the value of the field grouping by the value of another field. While it was easy to configure a bar chart that would display counts of features grouped by sum value, I could not create a bar chart that displayed a statistical calculation of one field grouped by another.
Use case example: I have a feature class of building footprints. Each footprint has an attribute storing total gross sqft and and attribute storing a classification value for the building. I wanted to create a bar chart that displayed the total gross sqft per building classification from a feature layer data source. While I could (and did eventually) create a table layer in my service that calculated total gross sqft per classifiction, the disadvantage to this approach was that the bar chart widget could not update dynamically in response to filters applied to the feature class layer.