[ingvarch]

Description

Adds a logarithmic Y scale for metrics that span orders of magnitude, such as latency, where a linear scale presses normal traffic against the bottom row.

• WithLogScale, SetLogScale and LogScale(). Values map to rows by log10; values at or below zero are drawn as gaps. Push, Last, SeriesData and legends keep the real values.
• Range fix-ups when drawing: a non-positive minimum becomes max / 10, both bounds non-positive give 1..10. Min() and Max() keep the stored range.
• Ticks at the powers of ten, with 2x and 5x values when fewer than two powers fit, thinned to the chart height (scale.LogTicks). Auto range widens to whole decades (scale.LogRange).
• Thresholds follow the same mapping; a threshold at or below zero draws nothing.
• Zero baseline, symmetric range, inverted series, negative styles and zero-crossing fill are ignored on a log scale; the docs say so. The fill goes down to the bottom row.
• Renderers get an unexported plot-space Chart (range 0..decades, values log10(v / lo)), so the Chart interface and the renderers do not change.
• New section "Logarithmic Scale" in docs/axes-and-grid.md with the logscale screenshot.

Bugs found and fixed along the way:

• SetSymmetric(true) after turning auto range off mirrored the range set before auto range, not the range last drawn.
• On amd64, math.Log reads the exponent from the bits, so Log10 of a subnormal value is off by many decades. The decade search now corrects the guess in a loop (CI on Ubuntu caught it).

Left for later:

• No Bubble Tea demo toggles the log scale.

Issues

Closes #10

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 ./...) and everything builds (go build ./... and go -C examples build ./...).
• I have updated any relevant documentation.