I´ve just realized that when a stroke gradient is added (via stroke:some-continuous-variable) the curve interpolation is somewhat ignored and linear curve is used. Is this a bug, a feature or am I doing something wrong?

It's a known limitation, mostly due to the fact that there is no easy way to create the shape with varying colors in SVG (in particular if the shape self-intersects).

Checking the API for the line mark's channels does not detail this behavior for a x or y channel value of 'Date'. As in the cell's result, both x and y channels do not display a label for a value of 'Date' and behave as though the channel was assigned: d => d.Date. Is 'Date' a special channel attribute value?

The logic you are looking for is here: https://github.com/observablehq/plot/blob/977e0118dad0f05158dbc3c0cab5f92047f449fd/src/axes.js#L173-L174 I can’t find whether we documented the default behavior of the scale.label option, but you can see it in the linked source above: if all the channels for a given scale are consistently labeled (i.e., have the same value if not undefined), and the corresponding axis doesn’t already have an explicit label, then the channels’ label is promoted to the corresponding axis. There is an exception for temporal scales when the default label would be “date”, “time”, or “year” (case-insensitive). You can override this default behavior by setting the label option on the scale.