[Feature] accessing legend unselected series in tooltip formatter function

### What problem does this feature solve?

https://echarts.apache.org/examples/en/editor.html?c=line-simple&code=PYBwLglsB2AEC8sDeAoWsAeBBDEDOAXMmurGAJ4gCmRA5AMYCGYVA5sAE7m0A0J6AE2aMiAbVoBZGL1i0AKgFcqM2gHUqAlXIAWClQDEOEFQGVmphdFoBdEgF8-6cjnxFUpMpRqyAbowA2SrT2jrD-bFTQAgTupABGwGBgwAC2BABEAEwApOmh6HhU4fQs0cQe6OkAjOlEAGYBhfywdiEkycD-kCAxzWBGrKxUHBmMuHh5zXWcKcwsIwAUjBwcAJTwAHyxHgJUcQqDw83o9DB4nVQAdP7ArEsrq82t6A4khUZUhLCizdukQmARN8qgBWAAMPFgmQAzBCoZkACyQzJVAAckKq0JBGIRAHZkQA2MHWfKkCjUOj-CDQZSQ6CMFLearpJ6kv6CYRiGFwzKI5FojFYnH4qFEknHTwU2RUmkyemMohZFkeZ6wWx2ADcQA

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in the tooltip formatter callback, `arr` contains only legend selected series, making it almost impossible to access unselected ones. I wanna achieve something like this, notice the series unselected by legend are still shown in the tooltip

accessing the unselected data through `getOption` is not viable here, as one can unselect all series, making it impossible to get data index through `arr[0].dataIndex`. and different series may have different data layout, we can't simply reuse dataIndex

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I did a lot of code reading but can't find where the `dataByCoordSys` is actually populated(checked `filterSeries` and related code but no luck
* can you point me the code path?

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### What does the proposed API look like?

modify the `formatter` callback signature like this, it should be mostly backward compatible
if concerned, we could add `toString` and `toJSON` to the new object, or just append a new parameter instead

or we we could do the following, it works according to ECMA standards although it may look unconventional