[charts] Allow SeriesValueFormatter to return null value

[clins1994]

TL;DR

A. Omit irrelevant tooltip values by updating the SeriesValueFormatter type to allow retuning null values
B. Usage: update valueFormatter function to return null if the value is irrelevant

Problem

Use case: when displaying multiple series with the same label/color on a stacked bar chart it can be useful to omit the series that have no value for a given stack.

Note: I am interested in seeing 'axis' tooltip not 'item'

const series = [
  { ... , valueFormatter: (v: number | null) => v ? `${v} 分` : '-' }
]

The screenshot below shows the tooltip of a stack that has "MOVING", "STOPPED", and "MOVING"

In this example the order of these events matter both on the x axis and y axis so it was decided to split the multiple "MOVING" data into separate series

When hovering over the tooltip it becomes polluted with irrelevant '-' values.

I am able to filter out the values by passing null as a return but I get a type error which I was able to ignore using !

const series = [
  { ... , valueFormatter: (v: number | null) => v ? `${v} 分` : null! }
]

With this I can visualize only the data that is displayed on the chart

Solution

Since forcing the return of null didn't break but actually fixed my issue I wrote this PR to update the type to match the already allowed and expected behavior

[mui-bot]

Deploy preview: https://deploy-preview-15057--material-ui-x.netlify.app/

Generated by 🚫 dangerJS against 6e5f596

[codspeed-hq]

CodSpeed Performance Report

Merging #15057 will not alter performance

_{Comparing clins1994:master (6e5f596) with master (018f118)}

Summary

✅ 3 untouched benchmarks

[JCQuintas]

Thanks @clins1994, this seems to be a good addition. @alexfauquette do you see any issue?

[alexfauquette]

LGTM 👍

I considered moving this to the fact that the value is null instead of the formatted one. But a user could have the opposite need: showing something in the tooltip for missing values

[clins1994]

@alexfauquette @JCQuintas thank you for the reviews 🤝