fix "axrange" edit under axis constraint

... by extracting correct prop "leaf" end index which
    is used to keep track of "altered ranges" which than
    lead to `flags.calc: true` under axis constraints.

    In the test added, the # of xgrid ended being the same
    before and after the relayout call.

Author: etpinard

src/plot_api/plot_api.js

@@ -1806,7 +1806,7 @@ function _relayout(gd, aobj) {
         var plen = p.parts.length;
         // p.parts may end with an index integer if the property is an array
         var pend = plen - 1;
-        while(pend > 0 && typeof p.parts[plen - 1] !== 'string') { pend--; }
+        while(pend > 0 && typeof p.parts[pend] !== 'string') pend--;
         // last property in chain (leaf node)
         var pleaf = p.parts[pend];
         // leaf plus immediate parent

test/jasmine/tests/cartesian_interact_test.js

@@ -554,6 +554,39 @@ describe('axis zoom/pan and main plot zoom', function() {
         .catch(failTest)
         .then(done);
     });
+
+    it('updates axis layout when the constraints require it', function(done) {
+        function _assert(xGridCnt) {
+            var xGrid = d3.select(gd).selectAll('.gridlayer > .x > path.xgrid');
+            expect(xGrid.size()).toEqual(xGridCnt);
+        }
+
+        Plotly.plot(gd, [{
+            x: [1, 1.5, 0, -1.5, -1, -1.5, 0, 1.5, 1],
+            y: [0, 1.5, 1, 1.5, 0, -1.5, -1, -1.5, 0],
+            line: {shape: 'spline'}
+        }], {
+            xaxis: {constrain: 'domain'},
+            yaxis: {scaleanchor: 'x'},
+            width: 700,
+            height: 500
+        })
+        .then(function() {
+            _assert(2);
+
+            return Plotly.relayout(gd, {
+                'xaxis.range[0]': 0,
+                'xaxis.range[1]': 1,
+                'yaxis.range[0]': 0,
+                'yaxis.range[1]': 1
+            });
+        })
+        .then(function() {
+            _assert(1);
+        })
+        .catch(failTest)
+        .then(done);
+    });
 });
 
 describe('Event data:', function() {

test/jasmine/tests/plot_api_test.js

@@ -669,6 +669,21 @@ describe('Test plot api', function() {
                 destroyGraphDiv();
             });
         });
+
+        it('should trigger calc on axis range updates when constraints are present', function() {
+            gd = mock({
+                data: [{
+                    y: [1, 2, 1]
+                }],
+                layout: {
+                    xaxis: {constrain: 'domain'},
+                    yaxis: {scaleanchor: 'x'}
+                }
+            });
+
+            Plotly.relayout(gd, 'xaxis.range[0]', 0);
+            expect(gd.calcdata).toBeUndefined();
+        });
     });
 
     describe('Plotly.restyle subroutines switchboard', function() {