XYChart2: Choose better series name for single dataset

[leeoniya]

x is sometimes shared, so we use the y field to identify each series, and usually take the name from it.

for datasets that do not contain frame names or labels to identify their "group" (e.g. from a transform), this doesnt really work well. this should probably be named generically "Series 1".

the tricky question here is how would users change this name (by override of Y field displayName) without also making the tooltip say

o MySeries
----------
Lat
My Series

internally, we use any common prefix and/or suffix of each y field as the series name, but when there is only one y field it becomes tricky. if we have a frame name that's different than y field name, we can use that, in this case it's the CSV file name:

we have some code in place for more advanced naming customization without resorting to manual mode but that's for future:

grafana/public/app/plugins/panel/xychart/v2/types2.ts

Lines 11 to 26 in cdb64cb

	// /*
	// replace?: {
	// // default: 'field'
	// source: 'field' | 'frame';
	// // default: whatever is matched for y field
	// matcher: common.MatcherConfig;
	// // default: 'displayName'
	// prop: 'displayName' | 'name' | 'query' | 'target';
	// // similar to renameByRegex & RenameByRegexTransformerOptions
	// // default: '(.*)'
	// regex: string;
	// // default: '$1'
	// rename: string;
	// }
	// */