The auto mark could be smarter and choose lineY

[mbostock]

Currently the auto mark tends to prefer line over lineX and lineY. There is no visible difference, but there is a difference in interaction behavior if we enable the default tip option as in #1546: with line, we get the two-dimensional pointer, but with lineY (the common choice for x = time and y = value) we would get the one-dimensional pointerX. Using the isMonotonic heuristic we should be able to prefer lineY or lineX in some cases and perhaps the same for areaY and areaX (we already prefer areaY).

[Fil]

The auto mark already uses the isMonotonic test on Y to select areaX, or else defaults to areaY (rather than area). Given this new use case with tip, and for consistency, we could apply the same logic to lines.

EDIT: I've tried with
markImpl = X && Y ? (yZero ? lineY : xZero || (Y && isMonotonic(Y)) ? lineX : lineY) : X ? lineX : lineY;
and that makes the tip frustrating on autoConnectedScatterplot.

But the following logic seems great:

      markImpl =
        X && Y
          ? yZero
            ? lineY
            : xZero || (Y && isMonotonic(Y))
            ? lineX
            : X && isMonotonic(X)
            ? lineY
            : line
          : X
          ? lineX
          : lineY; // 1d line by index