rename titleOffset -> Axes.getPxPosition

... which is a more appropriate name.
    Moreover, improve its perf by using
    _anchorAxis cache instead of `getFromId`

src/plots/cartesian/axes.js

@@ -1932,29 +1932,29 @@ axes.drawOne = function(gd, ax, opts) {
  push = {x: 0, y: 0, r: 0, l: 0, t: 0, b: 0};
 
  var bbox = ax._boundingBox;
- var titleOffset = getTitleOffset(gd, ax);
+ var pos = axes.getPxPosition(gd, ax);
  var anchorAxDomainIndex;
  var offset;
 
  switch(axLetter + s) {
  case 'xb':
  anchorAxDomainIndex = 0;
- offset = bbox.top - titleOffset;
+ offset = bbox.top - pos;
  push[s] = bbox.height;
  break;
  case 'xt':
  anchorAxDomainIndex = 1;
- offset = titleOffset - bbox.bottom;
+ offset = pos - bbox.bottom;
  push[s] = bbox.height;
  break;
  case 'yl':
  anchorAxDomainIndex = 0;
- offset = titleOffset - bbox.right;
+ offset = pos - bbox.right;
  push[s] = bbox.width;
  break;
  case 'yr':
  anchorAxDomainIndex = 1;
- offset = bbox.left - titleOffset;
+ offset = bbox.left - pos;
  push[s] = bbox.width;
  break;
  }
@@ -2658,14 +2658,28 @@ function drawDividers(gd, ax, opts) {
  .attr('d', opts.path);
 }
 
-function getTitleOffset(gd, ax) {
+/**
+ * Get axis position in px, that is the distance for the graph's
+ * top (left) edge for x (y) axes.
+ *
+ * @param {DOM element} gd
+ * @param {object} ax (full) axis object
+ * - {string} _id
+ * - {string} side
+ * if anchored:
+ * - {object} _anchorAxis
+ * Otherwise:
+ * - {number} position
+ * @return {number}
+ */
+axes.getPxPosition = function(gd, ax) {
  var gs = gd._fullLayout._size;
  var axLetter = ax._id.charAt(0);
  var side = ax.side;
  var anchorAxis;
 
  if(ax.anchor !== 'free') {
- anchorAxis = axisIds.getFromId(gd, ax.anchor);
+ anchorAxis = ax._anchorAxis;
  } else if(axLetter === 'x') {
  anchorAxis = {
  _offset: gs.t + (1 - (ax.position || 0)) * gs.h,
@@ -2683,7 +2697,7 @@ function getTitleOffset(gd, ax) {
  } else if(side === 'bottom' || side === 'right') {
  return anchorAxis._offset + anchorAxis._length;
  }
-}
+};
 
 function drawTitle(gd, ax) {
  var fullLayout = gd._fullLayout;
@@ -2699,8 +2713,7 @@ function drawTitle(gd, ax) {
  titleStandoff = 10 + fontSize * offsetBase + (ax.linewidth ? ax.linewidth - 1 : 0);
  }
 
- var titleOffset = getTitleOffset(gd, ax);
-
+ var pos = axes.getPxPosition(gd, ax);
  var transform, x, y;
 
  if(axLetter === 'x') {
@@ -2711,7 +2724,7 @@ function drawTitle(gd, ax) {
  } else {
  y = titleStandoff + fontSize * (ax.showticklabels ? 1.5 : 0.5);
  }
- y += titleOffset;
+ y += pos;
  } else {
  y = ax._offset + ax._length / 2;
 
@@ -2720,7 +2733,7 @@ function drawTitle(gd, ax) {
  } else {
  x = -titleStandoff - fontSize * (ax.showticklabels ? 0.5 : 0);
  }
- x += titleOffset;
+ x += pos;
 
  transform = {rotate: '-90', offset: 0};
  }