title: What is the meaning of attr("d", path)?
body: <p>I tried to find about 'd' in Official site...
  Maybe,, it's not related to D3.... 
   related more to javascript, css, svg, html....</p>
  
  <p>What is the meaning of    attr("d", path) ?</p>
  
  <pre><code>.....................
  var areas = group.append("path")
      .attr("d", path)
      .attr("class", "area")
      .attr("fill", "steelblue");
  </code></pre>
  
  <p>Is it different with  d in d3?</p>
  
  <pre><code>.style({
      'font-size':'18px',
      'padding':'6px',
      'margin':'4px',
      'list-style':'none',
      'background':function(d){
        return d.background;
      },
      'color':function(d){
        return d.color;
      },
      'width':function(d){
        return d.width+'%';
      }
    });
  </code></pre>