Waffles can overflow the frame

[mbostock]

As noted in #2132 (comment):

Plot.waffleY([0.5, 2, 4], {x: null, fill: Plot.indexOf}).plot({color: {type: "categorical"}})

Above, the y-scale only extends from [0, 6.5], but the waffle visually extends to y = 7.5… that’s because here each cell of the waffle is 3 y-units tall, as there are three columns. We’d need the waffle mark to provide a hint to the y scale that the domain needs to be taller than the apparent value from the y1 and y2 channels.