Automatically select the range for charts in a general way

Summary:
Ref T13279. Replace the hard-coded default range with a range computed by examining the chart data.

Instead of having a "Dataset" return a blob of wire data, "Dataset" now returns a structure with raw wire data plus a range. I expect to add more structured data here in future changes (tooltip/hover event data, maybe function labels).

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20503