[ingvarch]

Description

Adds a second Y axis on the right, so two metrics on different scales, such as CPU in percent and requests per second, share one chart.

• Axis{Min, Max, Ticks, Formatter}, WithRightAxis / SetRightAxis / RightAxis(), WithSeriesRightAxis / SetSeriesRightAxis / SeriesRightAxis(). NaN and infinite bounds are ignored, as in SetRange.
• Every line of View gets one cell for the right axis line plus a label column as wide as the widest right label. The plot stays w wide, the height does not change, and the time axis rows keep their alignment (the axis row ends with ┘).
• Right labels sit on the rows of their own ticks; the grid follows the left ticks.
• Legend and LegendBox mark right-axis series with →.
• Auto range, the zero baseline, the symmetric range, thresholds, threshold colors and negative styles stay on the left axis.
• View maps right-axis series into the left drawing space (also on a log scale) and gives renderers a wrapped Chart; RenderContext.RightAxis marks those series. The Chart interface does not change.
• New section "Right Axis" in docs/axes-and-grid.md with the rightaxis screenshot.

Decision on the open question:

• The right labels stay on their own tick rows instead of the left tick rows, so they stay round numbers.

Issues

Closes #12

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.