[wruza]

  y = 1 + ...

if y is 1-based too (which feels even more uncomfortable). But it would be nice to have such functions out if the box:

  canvas.linear_index(x, y, stride)
  vbox.y1_for(margin, spacing, height, i)
  vbox.y2_for(margin, spacing, height, i)

And then you have "integer at the center of a pixel" vs "integer at the top-left of a pixel" differences, which should be taken into account by these functions too. Stroke 1: y+0.5, y+height-1. Stroke 2: y+1, y+height-2. Fill: y, y+height.

Pixel geometry is hard on its own.