- fix d3.pointer for touch events

- add d3.pointers for multitouch events

closes #245

README.md

@@ -684,6 +684,10 @@ If the *target* is an HTML element, the event’s coordinates are translated rel
 
 Otherwise, [*event*.pageX, *event*.pageY] is returned.
 
+<a name="pointers" href="#pointers">#</a> d3.<b>pointers</b>(<i>event</i>[, <i>target</i>]) [<>](https://github.com/d3/d3-selection/blob/master/src/pointer.js "Source")
+
+A generalization of [pointer](#pointer) for multitouch events. Returns the *x* and *y* coordinates of the specified *event*’s touches relative to the specified *target*, as an array of two-element arrays of numbers [[*x1, y1*], [*x2, y2*], …]. If *target* is not specified, it defaults to *event*.currentTarget.
+
 ### Control Flow
 
 For advanced usage, selections provide methods for custom control flow.

src/index.js

@@ -4,7 +4,7 @@ export {default as local} from "./local.js";
 export {default as matcher} from "./matcher.js";
 export {default as namespace} from "./namespace.js";
 export {default as namespaces} from "./namespaces.js";
-export {default as pointer} from "./pointer.js";
+export {default as pointer, pointers} from "./pointer.js";
 export {default as select} from "./select.js";
 export {default as selectAll} from "./selectAll.js";
 export {default as selection} from "./selection/index.js";

src/pointer.js

@@ -1,4 +1,5 @@
-export default function(event, node = event.currentTarget) {
+export default function pointer(event, node = event.currentTarget) {
+  if (event instanceof TouchEvent) event = event.touches[0];
   if (node) {
     var svg = node.ownerSVGElement || node;
     if (svg.createSVGPoint) {
@@ -14,3 +15,7 @@ export default function(event, node = event.currentTarget) {
   }
   return [event.pageX, event.pageY];
 }
+
+export function pointers(event, node = event.currentTarget) {
+  return Array.from(event.touches || [event]).map(e => pointer(e, node));
+}