making datum/index usage optional

d3/d3.d.ts

@@ -1727,7 +1727,7 @@ declare module D3 {
         export interface Symbol {
             type: (symbolType: string | ((datum: any, index: number) => string)) => Symbol;
             size: (size: number | ((datum: any, index: number) => number)) => Symbol;
-            (datum:any, index:number): string;
+            (datum?: any, index?: number): string;
         }
 
         export interface Brush {