demo of hover/cursor snapping. close #972.

Author: leeoniya

demos/nearest-non-null.html

@@ -339,14 +339,44 @@
 				];
 
 				let opts = {
-					title: 'Snap cursor to last',
+					title: 'Snap hover points (and/or cusror) to previous non-null',
 					width: 1000,
 					height: 500,
 					cursor: {
+						// can use this if only snapping hover points, but keeping cursor at mouse coords
 						hover: {
 							bias: -1,
 							skip: [null],
 						},
+					/*
+						// OR...
+
+						// this manually implements bias instead of relying on dataIdx/hover opts since they
+						// are invoked later (and currently depend on the output coords of cursor.move)
+						move: (u, mouseLeft, mouseTop) => {
+							let curXVal = u.posToVal(mouseLeft, 'x');
+							let curIdx  = u.valToIdx(curXVal);
+							let dataX   = u.data[0];
+							let dataY   = u.data[1];
+							let idxXVal = dataX[curIdx];
+							let idxYVal = dataY[curIdx];
+
+							// backward bias
+							if (curXVal < idxXVal || idxYVal == null) {
+								while (curIdx > 0) {
+									idxYVal = dataY[--curIdx];
+
+									if (idxYVal != null)
+										break;
+								}
+							}
+
+							return [
+								Math.round(u.valToPos(dataX[curIdx], 'x')),
+								mouseTop,
+							];
+						},
+					*/
 					},
 					scales: {
 						x: {