d3-interpolate-path is a D3 plugin that adds an interpolator optimized for SVG <path> elements.

Blog: Improving D3 Path Animation

Demo: http://peterbeshai.com/d3-interpolate-path/

var line = d3.line()
  .curve(d3.curveLinear)
  .x(function (d) { return x(d.x); })
  .y(function (d) { return y(d.y); });

d3.select('path.my-path')
  .transition()
  .duration(2000)
  .attrTween('d', function (d) {
    var previous = d3.select(this).attr('d');
    var current = line(d);
    return d3.interpolatePath(previous, current);
  });

If you're using it in a module environment, you can import it as follows:

import { interpolatePath } from 'd3-interpolate-path';

Get rollup watching for changes and rebuilding

npm run watch

Run a web server in the docs directory

cd docs
php -S localhost:8000

Go to http://localhost:8000

If you use NPM, npm install d3-interpolate-path. Otherwise, download the latest release.

# interpolatePath(a, b, excludeSegment)

Returns an interpolator between two path attribute d strings a and b. The interpolator extends a and b to have the same number of points before using d3.interpolateString on them.

var pathInterpolator = interpolatePath('M0,0 L10,10', 'M10,10 L20,20 L30,30')
pathInterpolator(0)   // 'M0,0 L10,10 L10,10'
pathInterpolator(0.5) // 'M5,5 L15,15 L20,20'
pathInterpolator(1)   // 'M10,10 L20,20 L30,30'

You can optionally provide a function excludeSegment that takes two adjacent path commands and returns true if that segment should be excluded when splitting the line. A command object has form { type, x, y } (with possibly more attributes depending on type). An example object:

// equivalent to M0,150 in a path `d` string
{
  type: 'M',
  x: 0,
  y: 150
}

This is most useful when working with d3-area. Excluding the final segment (i.e. the vertical line at the end) from being split ensures a nice transition. If you know that highest x value in the path, you can exclude the final segment by passing an excludeSegment function similar to:

function excludeSegment(a, b) {
  return a.x === b.x && a.x === 300; // here 300 is the max X
}