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.

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) {