further tweak of autoShiftNumericBins

and fix its tests for the improved behavior

Author: alexcjohnson

src/plots/cartesian/axes.js

@@ -610,7 +610,8 @@ function autoShiftNumericBins(binStart, data, ax, dataMin, dataMax) {
         // the bin size, just enough to clear up endpoint
         // ambiguity about which integers are in which bins.
         else {
-            binStart += (binStart + 0.5 < dataMin) ? 0.5 : -0.5;
+            binStart -= 0.5;
+            if(binStart + ax.dtick < dataMin) binStart += ax.dtick;
         }
     }
     else if(midcount < dataCount * 0.1) {

test/jasmine/tests/axes_test.js

@@ -1809,7 +1809,7 @@ describe('Test axes', function() {
             );
 
             expect(out).toEqual({
-                start: -0.5,
+                start: 0.5,
                 end: 4.5,
                 size: 1
             });
@@ -1822,6 +1822,9 @@ describe('Test axes', function() {
                 2
             );
 
+            // when size > 1 with all integers, we want the starting point to be
+            // a half integer below the round number a tick would be at (in this case 0)
+            // to approximate the half-open interval [) that's commonly used.
             expect(out).toEqual({
                 start: -0.5,
                 end: 5.5,