edge case in autoShiftNumericBins

avoid an empty bin at the start. Tested via histogram_test

Author: alexcjohnson

src/plots/cartesian/axes.js

@@ -609,7 +609,9 @@ function autoShiftNumericBins(binStart, data, ax, dataMin, dataMax) {
         // otherwise start half an integer down regardless of
         // the bin size, just enough to clear up endpoint
         // ambiguity about which integers are in which bins.
-        else binStart -= 0.5;
+        else {
+            binStart += (binStart + 0.5 < dataMin) ? 0.5 : -0.5;
+        }
     }
     else if(midcount < dataCount * 0.1) {
         if(edgecount > dataCount * 0.3 ||