fix handling of default tick0 with dtick for date axes

Author: alexcjohnson

src/plots/cartesian/layout_attributes.js

@@ -138,7 +138,6 @@ module.exports = {
     },
     tick0: {
         valType: 'any',
-        dflt: 0,
         role: 'style',
         description: [
             'Sets the placement of the first tick on this axis.',

src/plots/cartesian/tick_value_defaults.js

@@ -60,16 +60,17 @@ module.exports = function handleTickValueDefaults(containerIn, containerOut, coe
 
         // tick0 can have different valType for different axis types, so
         // validate that now. Also for dates, change milliseconds to date strings
-        var tick0 = coerce('tick0');
+        var tick0Dflt = (axType === 'date') ? '2000-01-01' : 0;
+        var tick0 = coerce('tick0', tick0Dflt);
         if(axType === 'date') {
-            containerOut.tick0 = Lib.cleanDate(tick0, '2000-01-01');
+            containerOut.tick0 = Lib.cleanDate(tick0, tick0Dflt);
         }
         // Aside from date axes, dtick must be numeric; D1 and D2 modes ignore tick0 entirely
         else if(isNumeric(tick0) && dtick !== 'D1' && dtick !== 'D2') {
             containerOut.tick0 = Number(tick0);
         }
         else {
-            containerOut.tick0 = 0;
+            containerOut.tick0 = tick0Dflt;
         }
     }
     else {

test/jasmine/tests/axes_test.js

@@ -693,6 +693,13 @@ describe('Test axes', function() {
             expect(axOut.tick0).toBe(someDate);
             expect(axOut.dtick).toBe('M15');
 
+            // dtick without tick0: get the right default
+            axIn = {dtick: 'M12'};
+            axOut = {};
+            mockSupplyDefaults(axIn, axOut, 'date');
+            expect(axOut.tick0).toBe('2000-01-01');
+            expect(axOut.dtick).toBe('M12');
+
             // now some stuff that shouldn't work, should give defaults
             [
                 ['next thursday', -1],