D3.js understand gradient fill

Question

I,m new to D3.js and trying to understand gradient fill in D3.js. I,m following this tutorial

http://bl.ocks.org/mbostock/1086421

and its working like a charm.

I just need someone to explain me whats happening in this code here :

var gradient = svg.append("defs")


 .append("linearGradient")
    .attr("id", "gradient")
    .attr("x1", "0%")
    .attr("y1", "0%")
    .attr("x2", "100%")
    .attr("y2", "100%")
    .attr("spreadMethod", "pad");

gradient.append("stop")
    .attr("offset", "0%")
    .attr("stop-color", "#0c0")
    .attr("stop-opacity", 1);

gradient.append("stop")
    .attr("offset", "100%")
    .attr("stop-color", "#c00")
    .attr("stop-opacity", 1);

svg.append("rect")
    .attr("width", width)
    .attr("height", height)
    .style("fill", "url(#gradient)");

From what i understand is that we are defining our gradient using the def tag. but i need to understand what the hell is going on in the rest of lines.

1)- What is appending this tag "linearGradient" has to do. Is that a valid html tag or can we create some other tag?

2)- What is speadMethod, "pad" doing ? What kind of attr is that ?

3)- Finally what is offset here ? what is stop-color and stop-opacity ?

An explanation in simple terms would be awesome so that i can change the color opacity according to my needs.

Answer 1

All of the code is performing work on a pre-existing <svg> element. The first statement adds a <defs> element to the svg element ( see more here ), and the subsequent gradient.append statements add specifics for a linear gradient, identified by #gradient which can be reused. The last statement creates a <rect> svg element and applies the #gradient def to it, utilizing this newly-created linear gradient definition.