Pie chart's .nv-label needs a pointer-events:none rule.

This makes tooltips stable and not disappear when you mouse over a label.
Issue #48

src/nv.d3.css

@@ -311,7 +311,6 @@ svg .title {
 
 .nvd3.nv-pie path {
   stroke-opacity: 0;
-
   transition: fill-opacity 250ms linear, stroke-width 250ms linear, stroke-opacity 250ms linear;
   -moz-transition: fill-opacity 250ms linear, stroke-width 250ms linear, stroke-opacity 250ms linear;
   -webkit-transition: fill-opacity 250ms linear, stroke-width 250ms linear, stroke-opacity 250ms linear;
@@ -331,12 +330,10 @@ svg .title {
 
 .nvd3.nv-pie .hover path {
   fill-opacity: .7;
-/*
-  stroke-width: 6px;
-  stroke-opacity: 1;
-*/
 }
-
+.nvd3.nv-pie .nv-label {
+  pointer-events: none;
+}
 .nvd3.nv-pie .nv-label rect {
   fill-opacity: 0;
   stroke-opacity: 0;