Display hover label at cursor position as fallback for scatter traces.

For curved edges of fills there is a chance that the hover detection polygons miss a point and the correct hover label position cannot
be determined. Previous code fell back to positioning the label
at the largest edge of any polygon of the trace of their combined
vertical midpoint, with undetermined behaviour if no polygon
overlapped the midpoint.
This change instead falls back to simply displaying the label at the
current cursor position, which simplifies code and results in less
confusing label placement.

src/traces/scatter/hover.js

@@ -151,16 +151,12 @@ module.exports = function hoverPoints(pointData, xval, yval, hovermode) {
         var xmax = -Infinity;
         var ymin = Infinity;
         var ymax = -Infinity;
-        var yminAll = Infinity;
-        var ymaxAll = -Infinity;
         var yPos;
 
         for(i = 0; i < polygons.length; i++) {
             var polygon = polygons[i];
             // This is not going to work right for curved or jagged edges, it will
             // act as though they're straight.
-            yminAll = Math.min(yminAll, polygon.ymin);
-            ymaxAll = Math.max(ymaxAll, polygon.ymax);
             if(polygon.contains(pt)) {
                 polygonsIn.push(polygon);
                 ymin = Math.min(ymin, polygon.ymin);
@@ -170,14 +166,9 @@ module.exports = function hoverPoints(pointData, xval, yval, hovermode) {
 
         // The above found no polygon that contains the cursor, but we know that
         // the cursor must be inside the fill as determined by the SVGElement
-        // (so we are probably close to a curved/jagged edge...). In this case
-        // as a crude approximation, simply consider all polygons for determination
-        // of the hover label position.
-        // TODO: This might cause some jumpiness of the label close to edges...
+        // (so we are probably close to a curved/jagged edge...).
         if(polygonsIn.length === 0) {
-            polygonsIn = polygons;
-            ymin = yminAll;
-            ymax = ymaxAll;
+            return null;
         }
 
         // constrain ymin/max to the visible plot, so the label goes
@@ -229,6 +220,18 @@ module.exports = function hoverPoints(pointData, xval, yval, hovermode) {
         if(inside) {
             var hoverLabelCoords = getHoverLabelPosition(trace._polygons);
 
+            // getHoverLabelPosition may return null if the cursor / hover point is not contained
+            // in any of the trace's polygons, which can happen close to curved edges. in that
+            // case we fall back to displaying the hover label at the cursor position.
+            if(hoverLabelCoords === null) {
+                hoverLabelCoords = {
+                    x0: pt[0],
+                    x1: pt[0],
+                    y0: pt[1],
+                    y1: pt[1]
+                };
+            }
+
             // get only fill or line color for the hover color
             var color = Color.defaultLine;
             if(Color.opacity(trace.fillcolor)) color = trace.fillcolor;