resolve #1988 - 🔪 ax._boundingBox (!)

- replace it with getLabelLevelBBox (an extension of getLabelLevelSpan),
  to compute axis margin push values. Use cache to not have to compute
  the (costly) label bounding boxes unnecessarily
- compute ax._depth during Axes.drawOne, which is the measure in px
  between axis position and outward-most part of bounding box,
  we use this for (1) multicategory label, (2) axis title and (3)
  rangeslider positioning

src/components/rangeslider/draw.js

@@ -108,15 +108,14 @@ module.exports = function(gd) {
 
         var gs = fullLayout._size;
         var domain = axisOpts.domain;
-        var tickHeight = opts._tickHeight;
 
         opts._width = gs.w * (domain[1] - domain[0]);
 
         var x = Math.round(gs.l + (gs.w * domain[0]));
 
         var y = Math.round(
             gs.t + gs.h * (1 - axisOpts._counterDomainMin) +
-            tickHeight +
+            (axisOpts.side === 'bottom' ? axisOpts._depth : 0) +
             opts._offsetShift + constants.extraPad
         );
 

src/components/rangeslider/helpers.js

@@ -43,17 +43,14 @@ exports.makeData = function(fullLayout) {
 
 exports.autoMarginOpts = function(gd, ax) {
     var opts = ax[name];
-
-    var tickHeight = (ax.side === 'bottom' && ax._boundingBox.height) || 0;
-    opts._tickHeight = tickHeight;
-
+    var bottomDepth = ax.side === 'bottom' ? ax._depth : 0;
     return {
         x: 0,
-        y: oppBottom,
+        y: ax._counterDomainMin,
         l: 0,
         r: 0,
         t: 0,
-        b: opts._height + gd._fullLayout.margin.b + tickHeight,
+        b: opts._height + gd._fullLayout.margin.b + bottomDepth,
         pad: constants.extraPad + opts._offsetShift * 2
     };
 };