fix #4027 - show zeroline even when no grid lines are present

- see #4027
  for complete rationale
- add assertZeroLines assertion

src/plots/cartesian/axes.js

@@ -2749,7 +2749,6 @@ axes.shouldShowZeroLine = function(gd, ax, counterAxis) {
         (rng[0] * rng[1] <= 0) &&
         ax.zeroline &&
         (ax.type === 'linear' || ax.type === '-') &&
-        ax._gridVals.length &&
         (
             clipEnds(ax, 0) ||
             !anyCounterAxLineAtZero(gd, ax, counterAxis, rng) ||

test/jasmine/tests/axes_test.js

@@ -3530,6 +3530,17 @@ describe('Test axes', function() {
             })
             .then(function() {
                 assertZeroLines(['x', 'y']);
+                return Plotly.relayout(gd, {
+                    'xaxis.showline': false, 'xaxis.nticks': 1, 'xaxis.range': [0, 0.1],
+                    'yaxis.showline': false, 'yaxis.nticks': 2, 'yaxis.range': [0, 0.1]
+                });
+            })
+            .then(function() {
+                // no grid lines, but still should show zeroline in this case
+                // see https://github.com/plotly/plotly.js/issues/4027
+                expect(gd._fullLayout.xaxis._gridVals.length).toBe(0, '# of grid lines');
+                expect(gd._fullLayout.xaxis._gridVals.length).toBe(0, '# of grid lines');
+                assertZeroLines(['x', 'y']);
             })
             .catch(failTest)
             .then(done);