Time axis

The Gantt's horizontal axis maps Date to pixel x. v1 uses a uniform-buckets approach; ISO-week-correct alignment is a deliberate deferral.

Mapping (v1)

For range = [range.start, range.end) and a plot area of plot_width pixels:

days_total = (range.end - range.start).num_days() as f32
day_index  = (date - range.start).num_days() as f32
x          = (day_index / days_total) * plot_width

This is uniform per day. A year with 365 days maps each day to plot_width / 365 px; a 366-day leap year maps to plot_width / 366 px. Bars across leap-year boundaries are slightly stretched.

Row mapping

For row index i (top to bottom):

y = header_height + i * theme.row_height + (theme.row_height - theme.bar_height) / 2

Bars are centred vertically within their row.

Grid line detection

The renderer walks range.start..range.end once per chart:

Emit a week divider at every Monday (ISO weekday 1).
Emit a month divider at every day 1.

Month dividers paint AFTER week dividers (heavier, on top).

Why not ISO-week-correct alignment in v1

Per the M-CHART.0 ticket:

v1 uses uniform 52-bucket division. Documented in time-axis.md.

Reasons:

Year-boundary handling is the hard part. ISO-week 53 doesn't always exist; ISO-week 1 can start in the previous calendar year. Getting it right adds branches without product value at this scope.
The 2026 fixture doesn't trip any ISO edge cases. v1's demo year is well-behaved; uniform buckets render visually-correct dividers.
The data API is unchanged. When a follow-on chunk ships ISO-correct alignment, no Gantt / Bar field changes — only the layout module's divider_dates function.

Future: zoom + scroll

A pannable / zoomable axis is M-CHART.5 (parking lot). The v1 chart is a single fixed-extent composition; interactivity ships later.