Timeline axis

The signature is that every year is measured against the same horizontal line. A rail puts one entry after another and the reader takes them in sequence, one at a time, with the previous one already scrolled past. An axis puts all five side by side and the reader compares them: 1889 against 2016, what the observatory looked at against what it looked through, in one glance. That is the thing a rail can never show, and it is why the copy in each column is written to be read against its neighbours rather than after them.

The axis is a structural mark carrying the whole composition, so it is currentColor and never an accent: it cannot resolve to nothing, it follows your text colour, and it inverts with the band when you recolour it.

How the line is kept continuous

There is no separator block and no rule element. Each era is a core/group inside a grid, and each of those groups carries its own border-top. The grid's column gap is zero, so the five segments butt and form one unbroken line rather than five dashes with holes between them: this is the failure to watch for, and any horizontal gap between the cells reintroduces it. The space between eras comes from each cell's own left and right padding, never from the gap. Each cell also carries a border-left, which drops from the line as a tick at the start of its era, so the year reads as a mark ON the axis rather than as a number under it. The cells are grid items, so they share a row height and the five ticks are the same length.

What happens as the page narrows

The eras sit in a grid layout with a minimumColumnWidth of 13rem rather than in a core/columns row, because five columns of readable copy do not fit in a tablet. Five across at a desktop measure, three and two at 820, and one per row on a phone. When they stack, each cell's top border becomes a per-entry rule and the left tick becomes a bracket down the side of the entry, which still reads as a dated record even though the parallel comparison is gone: five columns cannot hold at 390 and pretending otherwise would ladder the copy into ribbons two words wide. Every wrapped row keeps a continuous rule of its own, because the rule belongs to the cells and not to a wrapper.