Specify seriesGroup or dataGroup as dataGroup.
Specify comma-separated list for dataFields, (no need for !-prefixes).
When plotBy is "series" : the number of series equals the number of dataGroup elements found in the XML.
When plotBy is "item" : the number of series equals the number dataField entries specified.
If the dataGroup name is not unique, specify the context using XPath syntax. The default is to match
elements of that name anywhere (context="//").
Optionally specify a seriesNameField XPath expression. This is evaluated relative
to the dataGroup. If not specified, a default name is generated using
the dataGroup and a positional predicate.