[Feature] legend.formatter

### What problem does this feature solve?

```
legend.formatter(name) ----------> legend.formatter(name,serieName) or legend.formatter(name,serieOption)

```

### What does the proposed API look like?

```
legend.formatter(name,serieName)
```

or
```
series：{

legendFormatter(name){
}
}
```

Start by tracing the existing legend.formatter API and how legend labels are generated for different series. Compare the proposed formatter(name, serieName) and series.legendFormatter(name) options, then identify the expected behavior and coverage needed for the selected API before implementation.

Written by the indexing model from the issue text.