instituts = [... new Set(sondages.map(s => s.nom_institut))]
.map(i => ({
institut: i,
sondages: sondages.filter(s => s['nom_institut'] == i),
regions: [...new Set(sondages.filter(s => s['nom_institut'] == i).map(s => s.region))]
}))
.sort((a,b) => b.sondages.length - a.sondages.length)