'circle-color': { 'property': ‘level’, 'type': ‘exponential’, 'stops': [ [1, '#009900'], [100, '#1666ff'] }
'circle-color': { 'property': ‘density’, 'type': ‘interval’, 'stops': [ [25, '#009900'], [75, '#cc00ff'], [100, '#1666ff'] }
'circle-color': { 'property': 'type', 'type': 'categorical', 'stops': [ [‘low’, '#009900'], [‘medium’, '#cc00ff'], [‘high’, '#1666ff'] }
'circle-radius': { stops: [[8, 6], [16, 40]] }
'circle-radius’: { property: 'sqrt', stops: [ [0, 6], [250, 40] ] }
'circle-radius': { property: 'sqrt', stops: [ [{zoom: 8, value: 0}, 3], [{zoom: 8, value: 250}, 20], [{zoom: 16, value: 0}, 6], [{zoom: 16, value: 250}, 40] ] }