line.x()

line.x([x])

If x is specified, sets the x accessor to the specified function or number and returns this line generator. If x is not specified, returns the current x accessor, which defaults to:

function x(d) {
  return d[0];
}

When a line is generated, the x accessor will be invoked for each defined element in the input data array, being passed the element d, the index i, and the array data as three arguments. The default x accessor assumes that the input data are two-element arrays of numbers. If your data are in a different format, or if you wish to transform the data before rendering, then you should specify a custom accessor. For example, if x is a time scale and y is a linear scale:

var data = [
  {date: new Date(2007, 3, 24), value: 93.24},
  {date: new Date(2007, 3, 25), value: 95.35},
  {date: new Date(2007, 3, 26), value: 98.84},
  {date: new Date(2007, 3, 27), value: 99.92},
  {date: new Date(2007, 3, 30), value: 99.80},
  {date: new Date(2007, 4,  1), value: 99.47},
  …
];

var line = d3.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.value); });