add customPlot

```kotlin
val myPlot = customPlot {
layout {
theme(myTheme)
}
}

myPlot(data) {
...
}
```

Start by reviewing the existing plotting entry points and how layout themes and data are currently passed. Define the intended customPlot API from the example, including the expected behavior of layout { theme(myTheme) } and myPlot(data) { ... }. Done means the API design is agreed and the resulting behavior is covered by appropriate tests or examples.

Written by the indexing model from the issue text.