Fix bug with tooltip overrides

The `tooltip` var that was receiving the override tooltip function was
being assigned into `tooltip` - the DOM element - rather than `tooltip_`
- the function var.

d3.parsets.js

@@ -410,7 +410,7 @@
 
     parsets.tooltip = function(_) {
       if (!arguments.length) return tooltip;
-      tooltip = _ == null ? defaultTooltip : _;
+      tooltip_ = _ == null ? defaultTooltip : _;
       return parsets;
     };