[charts] Allow SeriesValueFormatter to return null value (@clins1994

) (mui#15295)

Co-authored-by: Caio Lins <[email protected]>

packages/x-charts/src/models/seriesType/common.ts

@@ -13,7 +13,7 @@ export type SeriesValueFormatterContext = {
 export type SeriesValueFormatter<TValue> = (
   value: TValue,
   context: SeriesValueFormatterContext,
-) => string;
+) => string | null;
 
 export type CommonSeriesType<TValue> = {
   id?: SeriesId;
@@ -22,7 +22,7 @@ export type CommonSeriesType<TValue> = {
    * Formatter used to render values in tooltip or other data display.
    * @param {TValue} value The series' value to render.
    * @param {SeriesValueFormatterContext} context The rendering context of the value.
-   * @returns {string} The string to display.
+   * @returns {string | null} The string to display or null if the value should not be shown.
    */
   valueFormatter?: SeriesValueFormatter<TValue>;
   /**