Fix date tooltip: show human-readable dates instead of ms timestamps

[Copilot]

When a Date/POSIXt column is mapped to x, the tooltip displayed raw millisecond epoch values (e.g. 1767484800000) instead of readable date strings — both in the tooltip title and in tooltip items for point marks.

Why not just send ISO strings?

The ms-timestamp conversion in annotate_df must stay. G2's inferDomainQ computes the scale domain via +d (JS numeric coercion), and +"2026-01-01" === NaN — so sending ISO strings collapses all points to a single position. This was the original fix in PR #21.

Fix

build_config now:

1. Tooltip title: Auto-injects a JS tooltip title formatter on each child mark when the x channel is a Date/POSIXt column and the mark doesn't already have a custom tooltip.title:

   • Date → (d) => new Date(d["col"]).toLocaleDateString()
   • POSIXt → (d) => new Date(d["col"]).toLocaleString()

2. Tooltip items for point marks: For marks that use tooltip2d (point, cell, heatmap, image, link, beeswarm), also injects a valueFormatter on the x channel item so the date appears formatted in the items list too, not just the title. User-set tooltip$items is respected.

3. UTC time scale: Adds utc: true to the auto-detected time scale. This switches D3 to its UTC-based time scale so tick positions are at UTC midnight — matching our UTC-based ms timestamps. Without this, in non-UTC timezones D3's local-time ticks were offset from the marks.

The column name is escaped via xfun::tojson() before interpolation. User-set tooltip$title and tooltip = FALSE are both respected.

# Tooltip title: shows "1/5/2026" instead of "1767484800000"
data.frame(date = seq(as.Date('2026-01-01'), as.Date('2026-01-10'), length.out = 10),
           value = 1:10) |>
  g2(value ~ date)

# Tooltip items for point marks also show formatted dates
data.frame(date = Sys.Date() + 1:10, value = 1:10) |>
  g2(value ~ date) |> mark_line() |> mark_point()

[yihui]

another problem is, tooltips for points still show timestamps:

data.frame(date = Sys.Date() + 1:10, value = 1:10) |> 
    g2(value ~ date) |> mark_line() |> mark_point()

when you hover over a point, the tooltip shows both date and value, and date is presented as large integers

i also discovered a different pre-existing problem, not related to this PR but very serious: the x-axis tick marks are not aligned with marks in the plot when x axis is date or time---the marks are off to the left of their corresponding tick marks, e.g., the mark for Jan 2 is to the left of the Jan 2 tickmark; i tried a normal plot with numeric x axis, and the plot marks aligned perfectly with axis tick marks:

data.frame(date = 1:10, value = 1:10) |> 
    g2(value ~ date) |> mark_line() |> mark_point()

pls do browser testing and provide screenshots as proof for your fix following copilot instructions, also wait for CI

[yihui]

.slice(0, 10) is not robust

Suggested change

	'(d) => new Date(d[', col_js, ']).toISOString().slice(0, 10)'))
	'(d) => new Date(d[', col_js, ']).toLocaleDateString()'))

[Copilot]

Done in 504b34b — changed to toLocaleDateString() for Date columns.

[yihui]

similarly:

Suggested change

	'(d) => new Date(d[', col_js, ']).toISOString()',
	'.replace("T", " ").slice(0, 19)'))
	'(d) => new Date(d[', col_js, ']).toLocaleString()'))

[Copilot]

Done in 504b34b — changed to toLocaleString() for POSIXt columns.

[yihui]

follow instructions! no news for v0.1! remember this forever!

[Copilot]

Removed in 504b34b. Sorry about that — won't happen again.

[Copilot]

another problem is, tooltips for points still show timestamps:

Fixed in 504b34b. Three things addressed:

1. Tooltip items for mark_point(): For marks that use G2's tooltip2d (point, cell, heatmap, etc.), the x channel now appears with a valueFormatter that calls toLocaleDateString(), so the date item shows 1/3/2026 instead of a raw ms number.

2. x-axis alignment: Added utc: true to the auto-detected time scale. This switches D3 to its UTC-based time scale, so tick positions are at UTC midnight — matching our UTC-based ms timestamps. In non-UTC timezones, D3's default local-time scale was placing ticks at local midnight while marks were at UTC midnight, causing the visible offset.

3. toLocaleDateString() / toLocaleString(): Applied both code review suggestions.

Screenshots showing the fixed mark_line() |> mark_point() tooltip (title 1/3/2026, items value: 3 and date: 1/3/2026, no ms anywhere) and correct mark/tick alignment:

Tooltip on point mark:

Line chart with correct tick alignment: