feature: default to y: 0(and x: 0) for marks #98

Sometimes we want to draw marks along the x or y axis, and it would be convenient to just define the channel that's encoding data and allow the other to default to 0.

For example, in this Dot Histogram example.

Converting to SveltePlot:

<Plot r={{ range: [0, 14] }}>
    <Dot
        {...binX(
            { data: olympians, x: 'weight' },
            { r: 'count' }
        )} />
</Plot>

Gives the error: "missing channel value: y".

(This is one of the issue surfaced by #72)