Adding a horizontal line similar to vertical line under mouse

Perhaps the owner when he returns can enable discussions. These aren't really PRs, but I find them useful and hopefully others will to

I added a feature to add horizontal lines to help supplement the vertical line under the mouse. Might not be the best way, very open to pointers.
```

function followMouse(cname) {
console.log("followmouse")
var horizontal = d3.select(cname)
.append("div")
.attr("class", "remove")
.style("position", "absolute")
.style("z-index", "9999")
.style("height", "1px")
.style("width", "600px")
.style("left", "10px")
.style("right", "30px")
.style("top", "0px")
.style("background", "#000")
.style("visibility", "hidden")

d3.select(cname)
.on("mousemove", function(){
var mousey = d3.mouse(this);
mousey = mousey[1] + 5;
if (mousey >4 && mousey < 150)
horizontal.style("top", mousey + "px" )
.style("visibility", "visible")})
.on("mouseover", function(){
var mousey = d3.mouse(this);
mousey = mousey[0] + 5;
if (mousey >4 && mousey < 150)
horizontal.style("top", mousey + "px")
.style("visibility", "visible")})
.on("mouseout", function(){
horizontal.style("visibility", "hidden")});
}
```