How to override a function in NVD3 library? - d3.js

Question

My goal is to override the function used by NVD3 for the content of a tooltip.
I found the function that provides this behaviour: contentGenerator.
Below here the code:
var contentGenerator = function(d) {
if (content != null) {
return content;
}
if (d == null) {
return '';
}
var table = d3.select(document.createElement("table"));
var theadEnter = table.selectAll("thead")
.data([d])
.enter().append("thead");
theadEnter.append("tr")
.append("td")
.attr("colspan",3)
.append("strong")
.classed("x-value",true)
.html(headerFormatter(d.value));
var tbodyEnter = table.selectAll("tbody")
.data([d])
.enter().append("tbody");
var trowEnter = tbodyEnter.selectAll("tr")
.data(function(p) { return p.series})
.enter()
.append("tr")
.classed("highlight", function(p) { return p.highlight});
trowEnter.append("td")
.classed("legend-color-guide",true)
.append("div")
.style("background-color", function(p) { return p.color});
trowEnter.append("td")
.classed("key",true)
.html(function(p) {return p.key});
trowEnter.append("td")
.classed("value",true)
.html(function(p,i) { return valueFormatter(p.value,i) });
trowEnter.selectAll("td").each(function(p) {
if (p.highlight) {
var opacityScale = d3.scale.linear().domain([0,1]).range(["#fff",p.color]);
var opacity = 0.6;
d3.select(this)
.style("border-bottom-color", opacityScale(opacity))
.style("border-top-color", opacityScale(opacity))
;
}
});
var html = table.node().outerHTML;
if (d.footer == undefined)
html += "<div class='footer'>" + d.footer + "</div>";
return html;
};
Now, I'd like to put inside my chart section the function to override this method.
var chart = nv.models.lineChart()
.x(function(d) { return d.x; })
.y(function(d) { return d.y; })
.useInteractiveGuideline(false)
.tooltipContent(function (key, x, y, e, graph) {
return contentGenerator
});
How Can I achieve this goal?
I wanted to customise:
1) In the header add a simple text
2) Add a footer