Single Stat and columns name

For a `type: 'single stat'`, when you add columns without standard names `_time` and `_value`, the graph displays the message `No latest value found`.
```
const table = Giraffe.newTable(3)
.addColumn('MyTimestamp', 'dateTime:RFC3339', 'time', [1589838401244, 1589838461244, 1589838521244])
.addColumn('Myvalues', 'double', 'number', [2.58, 7.11, 4.79])
```
This is useful for `Single stat on top of Line Layer` for example to have custom legends
Is it possible to add 'x' and 'y' in layer `SingleStatLayerConfig` ?

Start with the SingleStatLayerConfig entry point and the Giraffe.newTable example in the issue. Trace how a single stat selects time and value columns, then determine how configurable x and y fields should work for custom column names. Done means the example renders its latest value instead of “No latest value found” while preserving the standard _time and _value behavior.

Written by the indexing model from the issue text.