The answer for me was to not use symbols at all, just use paths like so:
// The amount of circles we want to make:
var count = 150;
// Create a symbol, which we will use to place instances of later:
// Place the instances of the symbol:
for (var i = 0; i < count; i++) {
// The center position is a random point in the view:
var path = new Path.Circle({
center: Point.random() * view.size,
radius: i / count + 0.5,
fillColor: 'white',
strokeColor: 'black'
});
if (i % 10 == 0)
path.style.fillColor = '#eee';
}
// The onFrame function is called up to 60 times a second:
function onFrame(event) {
// Run through the active layer's children list and change
// the position of the placed symbols:
for (var i = 0; i < count; i++) {
var item = project.activeLayer.children[i];
// Move the item 1/20th of its width to the right. This way
// larger circles move faster than smaller circles:
item.position.x += item.bounds.width / 300;
// If the item has left the view on the right, move it back
// to the left:
if (item.bounds.left > view.size.width) {
item.position.x = -item.bounds.width;
}
}
}