The stack transform should support BigInt

[Fil]

As a follow-up on #1279, we want this to work:

(it currently works if we use y2 instead of y, and skip the stack transform)

cc: @pstuffa

[Fil]

This is trickier than it seems. The crash happens in

plot/src/options.js

Line 94 in 4cfd2e8

return values instanceof type ? values.map(f) : type.from(values, f);

with type=Float64Array, and f being an accessor such as d => d["num"].

How could we know in advance that the accessor is going to return bigint, and that we need to wrap it with a Number() converter? Would it be ok to try/catch the error instead, or is it going to slow things down?