where datapoint:

@Serializable
data class DataPoint (
        val name: String,
        val value: Double
)

you ought to surround with ` ``` ` for ease of reading

I also tried:

@Serializable
data class DataPoint (
        val name: String,
        val value: String
)

@Serializable
data class DataPoints (
        val elements: MutableList<DataPoint>
)

with

val liste = DataPoints(elements = mutableListOf())
                        liste.elements.add(DataPoint("A", "1.34"))
                        val foo = JSON.stringify( liste)

ahhh

But it is for use in webview and I can't see how to do that without writing a full string parsing library…