use Axes.getPxPosition instead of ax._boundingBox

... to position cartesian spike edges. This allows us to not
    have to compute the axis bounding box to render the spikes.

Author: etpinard

src/components/fx/hover.js

@@ -605,15 +605,15 @@ function _hover(gd, evt, subplot, noHoverEvent) {
         var result = dragElement.unhoverRaw(gd, evt);
         if(hasCartesian && ((spikePoints.hLinePoint !== null) || (spikePoints.vLinePoint !== null))) {
             if(spikesChanged(oldspikepoints)) {
-                createSpikelines(spikePoints, spikelineOpts);
+                createSpikelines(gd, spikePoints, spikelineOpts);
             }
         }
         return result;
     }
 
     if(hasCartesian) {
         if(spikesChanged(oldspikepoints)) {
-            createSpikelines(spikePoints, spikelineOpts);
+            createSpikelines(gd, spikePoints, spikelineOpts);
         }
     }
 
@@ -1396,9 +1396,10 @@ function cleanPoint(d, hovermode) {
     return d;
 }
 
-function createSpikelines(closestPoints, opts) {
+function createSpikelines(gd, closestPoints, opts) {
     var container = opts.container;
     var fullLayout = opts.fullLayout;
+    var gs = fullLayout._size;
     var evt = opts.event;
     var showY = !!closestPoints.hLinePoint;
     var showX = !!closestPoints.vLinePoint;
@@ -1433,8 +1434,7 @@ function createSpikelines(closestPoints, opts) {
         var yMode = ya.spikemode;
         var yThickness = ya.spikethickness;
         var yColor = ya.spikecolor || dfltHLineColor;
-        var yBB = ya._boundingBox;
-        var xEdge = ((yBB.left + yBB.right) / 2) < hLinePointX ? yBB.right : yBB.left;
+        var xEdge = Axes.getPxPosition(gd, ya);
         var xBase, xEndSpike;
 
         if(yMode.indexOf('toaxis') !== -1 || yMode.indexOf('across') !== -1) {
@@ -1507,8 +1507,7 @@ function createSpikelines(closestPoints, opts) {
         var xMode = xa.spikemode;
         var xThickness = xa.spikethickness;
         var xColor = xa.spikecolor || dfltVLineColor;
-        var xBB = xa._boundingBox;
-        var yEdge = ((xBB.top + xBB.bottom) / 2) < vLinePointY ? xBB.bottom : xBB.top;
+        var yEdge = Axes.getPxPosition(gd, xa);
         var yBase, yEndSpike;
 
         if(xMode.indexOf('toaxis') !== -1 || xMode.indexOf('across') !== -1) {