[ingvarch]

Description

Pulse can now draw history charts, such as Azure Monitor CPU and memory over 24 hours or 7 days, for azt: missing points show as gaps, the grid no longer hides the line, and a time axis can go under the plot. Each commit is one of these changes.

• Gaps for missing points. NaN, +Inf and -Inf draw as gaps in both renderers: no line, no fill, no connecting stroke. The braille spline runs per segment, so it does not curve across a gap. Last skips trailing gaps, so LegendBox shows the last finite value. Before, Push(math.NaN()) drew a spike to the top row on arm64, a dot on the bottom row in braille mode, and on amd64 View() panicked with index out of range [-9223372036854775808]. ±Inf was clamped to the range edges; it is now a gap too.
• Grid at the tick rows only. Horizontal grid lines are drawn only on rows with a Y-axis label, as the README and docs/axes-and-grid.md already said. The code drew one on every row since the first commit, and a test asserted it; that test now checks the documented behavior. A series drawn with ─ no longer blends into the grid. All screenshots are regenerated.
• Time axis. WithXAxis(func(age int) string) and SetXAxis add an axis row and a label row under the plot. Age 0 is the newest point. Labels sit at the left edge, the quarter columns (the same columns as the vertical grid, marked with ┴) and the right edge. A label that does not fit, or would touch a neighbor, is dropped. Height() stays the plot height, and View() returns two more lines. New screenshot assets/xaxis.png.

Fixed along the way: after the grid change, a NaN tick in WithTicks indexed the grid out of range and panicked on amd64. Non-finite ticks are now skipped.

Without gaps and without the new option, the output is byte-identical to main except for the grid rows. This was checked on 34 charts in both modes. The tests also pass on linux/amd64.

Left for later:

• WithRange(NaN, x) breaks rendering: lines mode panics and braille hangs. The root cause is that SetRange accepts NaN.
• BrailleRenderer does not trim data longer than the width. A custom Chart that returns more points than Width() can draw a stray mark in column 0. Model always trims, so it is not affected.
• go.mod requires bubbletea and gopsutil only for examples/, so every module that imports pulse gets them in its module graph. A separate examples/go.mod would remove them.
• golangci-lint reports 5 findings that predate this branch: 4 SA4023 and 1 ineffassign.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement / Refactoring

Checklist

• I have read the CONTRIBUTING.md guide.
• My code adheres to the project's code style and conventions (gofmt, go vet).
• I have added tests that prove my fix is effective or that my feature works.
• All existing and new tests pass (go test -race ./...).
• I have updated any relevant documentation.