Improved hover detection for scatter plot fill tonext*

draftlogs/6865_fix.md

@@ -0,0 +1 @@
+- Improved hover detection for for scatter plot fill tonext* [[#6865](https://github.com/plotly/plotly.js/pull/6865)], with thanks to @lumip for the contribution!

src/traces/scatter/hover.js

@@ -119,64 +119,118 @@ module.exports = function hoverPoints(pointData, xval, yval, hovermode) {
         }
     }
 
-    // even if hoveron is 'fills', only use it if we have polygons too
-    if(hoveron.indexOf('fills') !== -1 && trace._polygons) {
-        var polygons = trace._polygons;
+    function isHoverPointInFillElement(el) {
+        // Uses SVGElement.isPointInFill to accurately determine wether
+        // the hover point / cursor is contained in the fill, taking
+        // curved or jagged edges into account, which the Polygon-based
+        // approach does not.
+        if(!el) {
+            return false;
+        }
+        var svgElement = el.node();
+        try {
+            var domPoint = new DOMPoint(pt[0], pt[1]);
+            return svgElement.isPointInFill(domPoint);
+        } catch(TypeError) {
+            var svgPoint = svgElement.ownerSVGElement.createSVGPoint();
+            svgPoint.x = pt[0];
+            svgPoint.y = pt[1];
+            return svgElement.isPointInFill(svgPoint);
+        }
+    }
+
+    function getHoverLabelPosition(polygons) {
+        // Uses Polygon s to determine the left- and right-most x-coordinates
+        // of the subshape of the fill that contains the hover point / cursor.
+        // Doing this with the SVGElement directly is quite tricky, so this falls
+        // back to the existing relatively simple code, accepting some small inaccuracies
+        // of label positioning for curved/jagged edges.
+        var i;
         var polygonsIn = [];
-        var inside = false;
         var xmin = Infinity;
         var xmax = -Infinity;
         var ymin = Infinity;
         var ymax = -Infinity;
-
-        var i, j, polygon, pts, xCross, x0, x1, y0, y1;
+        var yPos;
 
         for(i = 0; i < polygons.length; i++) {
-            polygon = polygons[i];
-            // TODO: this is not going to work right for curved edges, it will
-            // act as though they're straight. That's probably going to need
-            // the elements themselves to capture the events. Worth it?
+            var polygon = polygons[i];
+            // This is not going to work right for curved or jagged edges, it will
+            // act as though they're straight.
             if(polygon.contains(pt)) {
-                inside = !inside;
-                // TODO: need better than just the overall bounding box
                 polygonsIn.push(polygon);
                 ymin = Math.min(ymin, polygon.ymin);
                 ymax = Math.max(ymax, polygon.ymax);
             }
         }
 
-        if(inside) {
-            // constrain ymin/max to the visible plot, so the label goes
-            // at the middle of the piece you can see
-            ymin = Math.max(ymin, 0);
-            ymax = Math.min(ymax, ya._length);
-
-            // find the overall left-most and right-most points of the
-            // polygon(s) we're inside at their combined vertical midpoint.
-            // This is where we will draw the hover label.
-            // Note that this might not be the vertical midpoint of the
-            // whole trace, if it's disjoint.
-            var yAvg = (ymin + ymax) / 2;
-            for(i = 0; i < polygonsIn.length; i++) {
-                pts = polygonsIn[i].pts;
-                for(j = 1; j < pts.length; j++) {
-                    y0 = pts[j - 1][1];
-                    y1 = pts[j][1];
-                    if((y0 > yAvg) !== (y1 >= yAvg)) {
-                        x0 = pts[j - 1][0];
-                        x1 = pts[j][0];
-                        if(y1 - y0) {
-                            xCross = x0 + (x1 - x0) * (yAvg - y0) / (y1 - y0);
-                            xmin = Math.min(xmin, xCross);
-                            xmax = Math.max(xmax, xCross);
-                        }
+        // 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...).
+        if(polygonsIn.length === 0) {
+            return null;
+        }
+
+        // constrain ymin/max to the visible plot, so the label goes
+        // at the middle of the piece you can see
+        ymin = Math.max(ymin, 0);
+        ymax = Math.min(ymax, ya._length);
+
+        yPos = (ymin + ymax) / 2;
+
+        // find the overall left-most and right-most points of the
+        // polygon(s) we're inside at their combined vertical midpoint.
+        // This is where we will draw the hover label.
+        // Note that this might not be the vertical midpoint of the
+        // whole trace, if it's disjoint.
+        var j, pts, xAtYPos, x0, x1, y0, y1;
+        for(i = 0; i < polygonsIn.length; i++) {
+            pts = polygonsIn[i].pts;
+            for(j = 1; j < pts.length; j++) {
+                y0 = pts[j - 1][1];
+                y1 = pts[j][1];
+                if((y0 > yPos) !== (y1 >= yPos)) {
+                    x0 = pts[j - 1][0];
+                    x1 = pts[j][0];
+                    if(y1 - y0) {
+                        xAtYPos = x0 + (x1 - x0) * (yPos - y0) / (y1 - y0);
+                        xmin = Math.min(xmin, xAtYPos);
+                        xmax = Math.max(xmax, xAtYPos);
                     }
                 }
             }
+        }
+
+        // constrain xmin/max to the visible plot now too
+        xmin = Math.max(xmin, 0);
+        xmax = Math.min(xmax, xa._length);
+
+        return {
+            x0: xmin,
+            x1: xmax,
+            y0: yPos,
+            y1: yPos,
+        };
+    }
 
-            // constrain xmin/max to the visible plot now too
-            xmin = Math.max(xmin, 0);
-            xmax = Math.min(xmax, xa._length);
+    // even if hoveron is 'fills', only use it if we have a fill element too
+    if(hoveron.indexOf('fills') !== -1 && trace._fillElement) {
+        var inside = isHoverPointInFillElement(trace._fillElement) && !isHoverPointInFillElement(trace._fillExclusionElement);
+
+        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;
@@ -189,10 +243,10 @@ module.exports = function hoverPoints(pointData, xval, yval, hovermode) {
                 // never let a 2D override 1D type as closest point
                 // also: no spikeDistance, it's not allowed for fills
                 distance: pointData.maxHoverDistance,
-                x0: xmin,
-                x1: xmax,
-                y0: yAvg,
-                y1: yAvg,
+                x0: hoverLabelCoords.x0,
+                x1: hoverLabelCoords.x1,
+                y0: hoverLabelCoords.y0,
+                y1: hoverLabelCoords.y1,
                 color: color,
                 hovertemplate: false
             });