[Feature] Restart line smoothing at a marked vertex #21796

What problem does this feature solve?

One line series sometimes has a real join in the middle: history and a forecast, or a change of regime. Smoothing should not borrow slope from the other side of that join, but the stroke should stay one series so the legend and the axis tooltip do not split.

Two series with the same color can fake the corner, and they show up as two legend entries and two tooltip rows. null already opens a gap (connectNulls: false). markLine only draws a guide. This is a corner, not a gap.

There is no open issue asking for this. #7158 and #21532 are about unwanted corners, and #1817 asked to smooth across gaps. This asks for a deliberate corner.

What does the proposed API look like?

series: [{
    type: 'line',
    smooth: true,
    data: [
        20, 55, 30,
        { value: 80, smoothBreak: true },
        25, 60, 35, 70
    ]
}]

The stroke stays connected through that sample. Smoothing restarts there. smoothBreak is only read from a series.data object. A number, an [x, y] tuple, or a dataset row cannot carry it. Perhaps another name is clearer?