Sankey Gradients - Missing Gradient Solution

a solution 🎉

specifically, this stackoverflow answer has the workaround to solve this apparent bug in Chromium's implementation of the SVG 1.1 standard

in d3.sankey.js, we want to alter the return value of the path generator to ensure that we never return perfectly straight paths. inserting this this new moveto command "M" + -10 + "," + -10 on the first line does just that:

return "M" + -10 + "," + -10
     + "M" + x0  + "," + y0
     + "C" + x2  + "," + y0
     + " " + x3  + "," + y1
     + " " + x1  + "," + y1;

an iteration on by Patient Flow Sankey Particles from @micahstubbs

see also the earlier version with 13 layout iterations that happens to avoid any perfectly straight paths.

and also this earlier bug reproduction example with 14 Sankey layout iterations that does produce a couple of those problematic-for-Chromium perfectly straight SVG paths

inspired by the blog post Data-based and unique gradients for visualizations with d3.js and associated example Data based gradients - Simple - Solar system from @nadiehbremer

<!DOCTYPE html>
<meta charset='utf-8'>
<title>Sankey Gradients</title>
<script src='https://cdnjs.cloudflare.com/ajax/libs/d3/4.4.0/d3.min.js'></script>
<script src='d3.sankey.js'></script>
<script src='https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.19.0/babel.min.js'></script>
<style>
.node rect {
  cursor: move;
  fill-opacity: .9;
  shape-rendering: crispEdges;
}
 
.node text {
  pointer-events: none;
  font-family: Helvetica;
  font-size: 12px;
}
</style>
<body>
<div id='chart'>
<script lang='babel' type='text/babel'>
const units = '';
const margin = {top: 10, right: 10, bottom: 10, left: 10};
const width = 960 - margin.left - margin.right;
const height = 300 - margin.top - margin.bottom;

// zero decimal places
const formatNumber = d3.format(',.0f');

const format = d => `${formatNumber(d)} ${units}`;

const color = d3.scaleOrdinal()
  .domain([
    'All referred patients',
    'First consult outpatient clinic',
    'OR-receipt',
    'Start surgery',
    // 'No OR-receipt',
    // 'No emergency',
    // 'No surgery',
    'Emergency'
  ])
  .range([
    '#90eb9d',
    '#f9d057',
    '#f29e2e',
    '#00ccbc',
    '#d7191c'
  ]);

d3.select('#chart')
  .style('visibility', 'visible');

// append the svg canvas to the page
const svg = d3.select('#chart').append('svg')
  .attr('width', width + margin.left + margin.right)
  .attr('height', height + margin.top + margin.bottom)
  .append('g')
    .attr('transform', `translate(${margin.left},${margin.top})`);

// set the sankey diagram properties
const sankey = d3.sankey()
  .nodeWidth(12)
  .nodePadding(10)
  .size([width, height]);

const path = sankey.link();

// append a defs (for definition) element to your SVG
const defs = svg.append('defs');

// load the data
d3.json('data.json', (error, graph) => {
  console.log('graph', graph);
  sankey
    .nodes(graph.nodes)
    .links(graph.links)
    .layout(14); // any value > 13 breaks the link gradient
 
  // add in the links
  const link = svg.append('g').selectAll('.link')
    .data(graph.links)
    .enter().append('path')
      .attr('class', 'link')
      .attr('d', path)
      .style('stroke-width', d => Math.max(1, d.dy))
      .style('fill', 'none')
      .style('stroke-opacity', 0.18)
      .sort((a, b) => b.dy - a.dy)
      .on('mouseover', function() {
        d3.select(this).style('stroke-opacity', 0.5);
      })
      .on('mouseout', function() {
        d3.select(this).style('stroke-opacity', 0.2);
      });
 
  // add the link titles
  link.append('title')
    .text(d => `${d.source.name} → ${d.target.name}\n${format(d.value)}`);
 
  // add in the nodes
  const node = svg.append('g').selectAll('.node')
    .data(graph.nodes)
    .enter().append('g')
      .attr('class', 'node')
      .attr('transform', d => `translate(${d.x},${d.y})`)
      .call(d3.drag()
        .subject(d => d)
        .on('start', function() { 
          this.parentNode.appendChild(this); })
        .on('drag', dragmove));
 
  // add the rectangles for the nodes
  node.append('rect')
    .attr('height', d => d.dy)
    .attr('width', sankey.nodeWidth())
    .style('fill', d => { 
      if(color.domain().indexOf(d.name) > -1){
        return d.color = color(d.name);  
      } else {
        return d.color = '#ccc';
      }
    })
    .append('title')
      .text(d => `${d.name}\n${format(d.value)}`);
 
  // add in the title for the nodes
  node.append('text')
    .attr('x', -6)
    .attr('y', d => d.dy / 2)
    .attr('dy', '.35em')
    .attr('text-anchor', 'end')
    .attr('transform', null)
    .text(d => d.name)
    .filter(d => d.x < width / 2)
      .attr('x', 6 + sankey.nodeWidth())
      .attr('text-anchor', 'start');

// add gradient to links
  link.style('stroke', (d, i) => {
    console.log('d from gradient stroke func', d);

// make unique gradient ids  
    const gradientID = `gradient${i}`;

const startColor = d.source.color;
    const stopColor = d.target.color;

console.log('startColor', startColor);
    console.log('stopColor', stopColor);

const linearGradient = defs.append('linearGradient')
        .attr('id', gradientID);

linearGradient.selectAll('stop') 
      .data([                             
          {offset: '10%', color: startColor },      
          {offset: '90%', color: stopColor }    
        ])                  
      .enter().append('stop')
      .attr('offset', d => {
        console.log('d.offset', d.offset);
        return d.offset; 
      })   
      .attr('stop-color', d => {
        console.log('d.color', d.color);
        return d.color;
      });

return `url(#${gradientID})`;
  })
 
// the function for moving the nodes
  function dragmove(d) {
    d3.select(this).attr('transform', 
      `translate(${d.x = Math.max(0, Math.min(width - d.dx, d3.event.x))},${d.y = Math.max(0, Math.min(height - d.dy, d3.event.y))})`);
    sankey.relayout();
    link.attr('d', path);
  }
});
</script>
</body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/4.4.0/d3.min.js

https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.19.0/babel.min.js