What does the double question mark means at the end of the type?

Double?? is shorthand notation for Optional<Optional<Double>>, which is simply a nested Optional. Optional is a generic enum, whose Wrapped value can actually be another Optional and hence you can create nested Optionals.

let optional = Optional.some(2)
let nestedOptional = Optional.some(optional)

The type of nestedOptional here is Int??.

For your specific example, item.first is Double??, since item itself is of type [Double?] and Array.first also returns an Optional, hence you get a nested Optional.

Your compactMap call on data achieves nothing, since you call it on the outer-array, whose elements are non-optional arrays themselves. To filter out the nil elements from the nested arrays, you need to map over data and then call compactMap inside the map.

let nonNilData = data.map { $0.compactMap { $0 } } // [[100, 35.6], [110, 42.56], [120, 48.4], [200]]