clearer docs on date histogram autobins

Author: alexcjohnson

src/plots/cartesian/axes.js

@@ -548,6 +548,10 @@ axes.autoBin = function(data, ax, nbins, is2d) {
     }
     else {
         // month ticks - should be the only nonlinear kind we have at this point.
+        // dtick (as supplied by axes.autoTick) only has nonlinear values on
+        // date and log axes, but even if you display a histogram on a log axis
+        // we bin it on a linear axis (which one could argue against, but that's
+        // a separate issue)
         if(dummyAx.dtick.charAt(0) === 'M') {
             binStart = autoShiftMonthBins(binStart, data, dummyAx.dtick, dataMin);
         }

test/jasmine/tests/histogram2d_test.js

@@ -92,6 +92,7 @@ describe('Test histogram2d', function() {
             // TODO: even though the binning is done on non-uniform bins,
             // the display makes them linear (using only y0 and dy)
             // Can we also make it display the bins with nonuniform size?
+            // see https://github.com/plotly/plotly.js/issues/360
             expect(out.y0).toBe('1970-01-01 03:00');
             expect(out.dy).toBe(365.25 * oneDay);