tickFormat

nv.addGraph( function() {
  var data = [
    {"key" : "Amount", "values" : [
      ["1406188303", 383],
      ["1407121032", 1176],
      ["1407121182", 799],
      ["1408066502", 0]
    ]},
    {"key" : "Count", "values" : [
      ["2014-07-24", 1],
      ["2014-08-04", 2],
      ["2014-08-15", 1]
    ]}
  ];
  var is_amount = false;

  var xval = function( d ) {
    if( is_amount ) {
      return new Date( d[0] * 1000 );
    } else {
      return d[0];
    }
  };
  var yval = function( d ) {
    return d[1];
  };
  var xlabel = 'Date';
  var ylabel = 'Total Count';

  var data = [( is_amount ? data[0] : data[1] )];
  var chart = nv.models.lineChart().margin( {left : 100} )
      .useInteractiveGuideline( true )
      .transitionDuration( 350 ) //how fast do you want the lines to transition?
      .showLegend( true )       //Show the legend, allowing users to turn on/off line series.
      .showYAxis( true )        //Show the y-axis
      .showXAxis( true )        //Show the x-axis
      .x( xval )
      .y( yval );

  chart.xAxis.axisLabel( xlabel ).tickFormat( function( d ) {
    return d3.time.format( '%Y-%m-%d' )( new Date( d ) ); //Here d is always -1 to 1, why?
  } );
  chart.yAxis.axisLabel( ylabel );

  d3.select( '#order_line svg' )   //Select the <svg> element you want to render the chart in.
      .datum( data )          //Populate the <svg> element with chart data...
      .call( chart );         //Finally, render the chart!

  nv.utils.windowResize( function() {
    chart.update();
  } );

  return chart;
} );