fix axis automargin push offset value

- previously, we consider the ticks + tick labels to start right
  on the edge of the axis line. This is incorrect, there's a slight
  pad between tick labels and ticks that must be taken into account
  to get the correct automargin push

Author: etpinard

src/plots/cartesian/axes.js

@@ -1940,27 +1940,53 @@ axes.drawOne = function(gd, ax, opts) {
     var hasRangeSlider = Registry.getComponentMethod('rangeslider', 'isVisible')(ax);
 
     function doAutoMargins() {
-        var push, rangeSliderPush;
+        var s = ax.side.charAt(0);
+        var push;
+        var rangeSliderPush;
 
         if(hasRangeSlider) {
             rangeSliderPush = Registry.getComponentMethod('rangeslider', 'autoMarginOpts')(gd, ax);
         }
         Plots.autoMargin(gd, rangeSliderAutoMarginID(ax), rangeSliderPush);
 
-        var s = ax.side.charAt(0);
         if(ax.automargin && (!hasRangeSlider || s !== 'b')) {
             push = {x: 0, y: 0, r: 0, l: 0, t: 0, b: 0};
 
-            if(axLetter === 'x') {
-                push.y = (ax.anchor === 'free' ? ax.position :
-                    ax._anchorAxis.domain[s === 't' ? 1 : 0]);
-                push[s] += ax._boundingBox.height;
-            } else {
-                push.x = (ax.anchor === 'free' ? ax.position :
-                    ax._anchorAxis.domain[s === 'r' ? 1 : 0]);
-                push[s] += ax._boundingBox.width;
+            var bbox = ax._boundingBox;
+            var counterAx = mainPlotinfo[counterLetter + 'axis'];
+            var anchorAxDomainIndex;
+            var offset;
+
+            switch(axLetter + s) {
+                case 'xb':
+                    anchorAxDomainIndex = 0;
+                    offset = bbox.top - counterAx._length - counterAx._offset;
+                    push[s] = bbox.height;
+                    break;
+                case 'xt':
+                    anchorAxDomainIndex = 1;
+                    offset = counterAx._offset - bbox.bottom;
+                    push[s] = bbox.height;
+                    break;
+                case 'yl':
+                    anchorAxDomainIndex = 0;
+                    offset = counterAx._offset - bbox.right;
+                    push[s] = bbox.width;
+                    break;
+                case 'yr':
+                    anchorAxDomainIndex = 1;
+                    offset = bbox.left - counterAx._length - counterAx._offset;
+                    push[s] = bbox.width;
+                    break;
             }
 
+            push[counterLetter] = ax.anchor === 'free' ?
+                ax.position :
+                ax._anchorAxis.domain[anchorAxDomainIndex];
+
+            if(push[s] > 0) {
+                push[s] += offset;
+            }
             if(ax.title.text !== fullLayout._dfltTitle[axLetter]) {
                 push[s] += ax.title.font.size;
             }