wip: more usable spiderfy in cluster

Co-authored-by: David Larlet <david@larlet.fr>

Author: yohanboniface

umap/static/umap/js/modules/rendering/layers/cluster.js

@@ -55,7 +55,7 @@ const MarkerCluster = Marker.extend({
     const zoom = this._map.getZoom()
     const angle = (index * step * Math.PI) / 180
     const progress = index / this._layers.length
-    const radius = maxRadius * (1 - progress) ** 1
+    const radius = maxRadius * (1 - progress) ** 0.4
     const x = radius * Math.cos(angle)
     const y = radius * Math.sin(angle)
     const point = this._map.project([center.lat, center.lng], zoom)