Have D3 not draw missing data in line charts using line.defined

missing_data.js

// rig example, in config.js
const path = line()
  .curve(curveMonotoneX)
  .x(d => x(d.date))
  .y(d => y(d.value))
  .defined(d => (d.value !== null) && (d.value !== -999));

// d3 generic example, from: https://bl.ocks.org/EmbraceLife/b8dda609b87a3bab7e952769fd028f53
var line = d3.line() 
    .defined(function(d) { return d; })
    .x(function(d) { return x(d.x); })
    .y(function(d) { return y(d.y); });

// more here: https://www.geeksforgeeks.org/d3-js-line-defined-method/
// "The d3.line.defined() method lets you specify whether
// there is data defined for a given data point or not. If
// this method returns false, this means that data point exists, else true.
// Syntax: d3.line.defined(data_point);