title: transitioning lines in d3.js
body: <p>Sorry if this question seems stupid, but it's stumped me for a couple days, so I would really appreciate any simple input from anyone! :)
  I'm trying to construct a line graph in d3.js that transitions between different datasets. However, whenever I hit the transition function, it selects the original line on the graph, AND the axes of the graph, and transitions all of it. I tried to do selectAll("svg:path") instead of selectAll("path") but it just returned a DOMException 12, indicating that the selection didn't exist.</p>
  
  <pre><code>//beginning of code to append axis to graph
  graph.append("svg:g").attr("class", "x axis")
  
  //draws the original line on the graph
  graph.append("svg:path").attr("d", line(dataset1));
  
  //line function
  var line = d3.svg.line()
          .x(function(d) {return x(d.age);})
          .y(function(d) {return y(d.freq);})
          .interpolate("basis");
  
  //transition function
  function transition(newData) {          
          graph.selectAll("path")
              .data(newData)
          .transition()
              .duration(2000)
              .attr("d", line(newData));
      }
  </code></pre>
  
  <p>Thanks for any responses! :)</p>