Question

I am amazed by the brevity of d3.js to visualize data. The complexity hidden under this brevity also makes it a bit hard to understand how it works. Below is part of the code from http://mbostock.github.com/d3/ex/calendar.html

The question is how it creates rect for day, within the number of svg created (var svg = d3….). In the statement svg.selectAll("rect.day") , I am not sure how it appends rect for each of svg (selectAll trying to select rect.day !)

var margin = {top: 19, right: 20, bottom: 20, left: 19},
    width = 960 - margin.right - margin.left, // width
    height = 136 - margin.top - margin.bottom, // height
    cellSize = 17; // cell size

var day = d3.time.format("%w"),
    week = d3.time.format("%U"),
    percent = d3.format(".1%"),
    format = d3.time.format("%Y-%m-%d");


var svg = d3.select("#chart").selectAll("svg")
    .data(d3.range(1990, 2011))
  .enter().append("svg")
    .attr("width", width + margin.right + margin.left)
    .attr("height", height + margin.top + margin.bottom)
    .attr("class", "RdYlGn")
  .append("g")
    .attr("transform", "translate(" + (margin.left + (width - cellSize * 53) / 2) + "," + (margin.top + (height - cellSize * 7) / 2) + ")");

var rect = svg.selectAll("rect.day")
    .data(function(d) { return d3.time.days(new Date(d, 0, 1), new Date(d + 1, 0, 1)); })
  .enter().append("rect")
    .attr("class", "day")
    .attr("width", cellSize)
    .attr("height", cellSize)
    .attr("x", function(d) { return week(d) * cellSize; })
    .attr("y", function(d) { return day(d) * cellSize; })
    .datum(format);

Answer 1

Have you looked at Thinking With Joins? It explains pretty clearly how data joins work – essentially, in D3, you use .selectAll() to join data to a selection, and then use .enter() to append new elements as necessary. So

svg.selectAll("rect.day")
    .data( ... )
  .enter().append("rect")
    .class("day")

creates new rect elements as necessary, based on the data.