avoid crashing on low curvatures (rho=1e-3 gives a good approximation…

… of a linear transform)

src/zoom.js

@@ -63,7 +63,7 @@ export default (function zoomRho(rho, rho2, rho4) {
   }
 
   zoom.rho = function(_) {
-    var _1 = +_, _2 = _1 * _1, _4 = _2 * _2;
+    var _1 = Math.max(1e-3, +_), _2 = _1 * _1, _4 = _2 * _2;
     return zoomRho(_1, _2, _4);
   };