Fix domain calculation where domain isn't set

[jamesrswift]

Fixes #72

Changes the default domain from (0, 0) to (none, none). It appears just to have been a typo that none of the tests picked up because they all included the origin.

Adjusted MRE (adjusted ticks)

#test-case({
  import draw: *
  set-style(rect: (stroke: none))

  plot.plot(size: (6, 4), x-tick-step: 0.5, {
    plot.add(domain: (100, 101), calc.sin)
    plot.annotate(padding: .1, {
      content( (101.5, 0), [A])
    })
  })
})

[johannes-wolf]

LGMT but I've created a follow up that includes the missing test file.

[johannes-wolf]

Closing in favor of #78