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 thisDot Histogram example.

Converting to SveltePlot:

<Plotr={{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)