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Create a gradient line using an expression

Use the line-gradient paint property and an expression to visualize distance from the starting point of a line.

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Create a gradient line using an expression</title>
    <meta property="og:description" content="Use the line-gradient paint property and an expression to visualize distance from the starting point of a line." />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/[email protected]/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/[email protected]/dist/maplibre-gl.js'></script>
    <style>
        body { margin: 0; padding: 0; }
        html, body, #map { height: 100%; }
    </style>
</head>
<body>
<div id="map"></div>

<script>
    const map = (window.map = new maplibregl.Map({
        container: 'map',
        style:
            'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL',
        center: [-77.035, 38.875],
        zoom: 12
    }));

    const geojson = {
        'type': 'FeatureCollection',
        'features': [
            {
                'type': 'Feature',
                'properties': {},
                'geometry': {
                    'coordinates': [
                        [-77.044211, 38.852924],
                        [-77.045659, 38.860158],
                        [-77.044232, 38.862326],
                        [-77.040879, 38.865454],
                        [-77.039936, 38.867698],
                        [-77.040338, 38.86943],
                        [-77.04264, 38.872528],
                        [-77.03696, 38.878424],
                        [-77.032309, 38.87937],
                        [-77.030056, 38.880945],
                        [-77.027645, 38.881779],
                        [-77.026946, 38.882645],
                        [-77.026942, 38.885502],
                        [-77.028054, 38.887449],
                        [-77.02806, 38.892088],
                        [-77.03364, 38.892108],
                        [-77.033643, 38.899926]
                    ],
                    'type': 'LineString'
                }
            }
        ]
    };

    map.on('load', () => {
        // 'line-gradient' can only be used with GeoJSON sources
        // and the source must have the 'lineMetrics' option set to true
        map.addSource('line', {
            type: 'geojson',
            lineMetrics: true,
            data: geojson
        });

        // the layer must be of type 'line'
        map.addLayer({
            type: 'line',
            source: 'line',
            id: 'line',
            paint: {
                'line-color': 'red',
                'line-width': 14,
                // 'line-gradient' must be specified using an expression
                // with the special 'line-progress' property
                'line-gradient': [
                    'interpolate',
                    ['linear'],
                    ['line-progress'],
                    0,
                    'blue',
                    0.1,
                    'royalblue',
                    0.3,
                    'cyan',
                    0.5,
                    'lime',
                    0.7,
                    'yellow',
                    1,
                    'red'
                ]
            },
            layout: {
                'line-cap': 'round',
                'line-join': 'round'
            }
        });
    });
</script>
</body>
</html>