fix #2892 - don't allow auto date ticks < our 100 microseconds limit

Author: alexcjohnson

src/plots/cartesian/axes.js

@@ -692,6 +692,9 @@ axes.autoTicks = function(ax, roughDTick) {
             base = getBase(10);
             ax.dtick = roundDTick(roughDTick, base, roundBase10);
         }
+        // ensure we don't try to make ticks below our minimum precision
+        // see https://github.com/plotly/plotly.js/issues/2892
+        if(ax.dtick < 0.1) ax.dtick = 0.1;
     }
     else if(ax.type === 'log') {
         ax.tick0 = 0;

test/jasmine/tests/axes_test.js

@@ -2057,6 +2057,19 @@ describe('Test axes', function() {
             expect(textOut).toEqual(expectedText);
         });
 
+        it('never gives date dtick < 100 microseconds', function() {
+            var ax = {
+                type: 'date',
+                tickmode: 'auto',
+                nticks: '100',
+                range: ['2017-02-08 05:21:18.145', '2017-02-08 05:21:18.1451']
+            };
+
+            var textOut = mockCalc(ax);
+            var expectedText = ['05:21:18.145<br>Feb 8, 2017', '05:21:18.1451'];
+            expect(textOut).toEqual(expectedText);
+        });
+
         it('should handle edge cases with dates and tickvals', function() {
             var ax = {
                 type: 'date',