fix the underlying problem that led to #970

even though it's hard to actually trigger it now that we're using
gradients for continuous colorscales (would require a very dense
contour set colored by fills)

Author: alexcjohnson

src/components/colorbar/draw.js

@@ -367,6 +367,12 @@ module.exports = function draw(gd, id) {
                 .classed(cn.cbfill, true)
                 .style('stroke', 'none');
             fills.exit().remove();
+
+            var zBounds = zrange
+                .map(cbAxisOut.c2p)
+                .map(Math.round)
+                .sort(function(a, b) { return a - b; });
+
             fills.each(function(d, i) {
                 var z = [
                     (i === 0) ? zrange[0] :
@@ -379,9 +385,7 @@ module.exports = function draw(gd, id) {
 
                 // offset the side adjoining the next rectangle so they
                 // overlap, to prevent antialiasing gaps
-                if(i !== filllevels.length - 1) {
-                    z[1] += (z[1] > z[0]) ? 1 : -1;
-                }
+                z[1] = Lib.constrain(z[1] + (z[1] > z[0]) ? 1 : -1, zBounds[0], zBounds[1]);
 
                 // Colorbar cannot currently support opacities so we
                 // use an opaque fill even when alpha channels present