The ggplot2 argument is linewidth -- should we instead go in the other direction to be consistent with the core plotting framework?

[strengejacke]

Fixes #452

[gemini-code-assist]

Code Review

This pull request introduces the linewidth argument as an alias for size_line across various plotting functions, updating the corresponding implementation, documentation, and tests. The reviewer points out an issue in plot.see_p_function where passing a single value via the alias can trigger a sanity check error when multiple groups are present, and suggests replicating the value to a length of 2 to improve robustness.

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[gemini-code-assist]

In plot.see_p_function, size_line is expected to be a vector of length 2 when there are multiple groups (to specify sizes for both regular and emphasized interval lines). If a user passes a single value via the linewidth alias (or size_line), it will trigger a sanity check error on line 75. Replicating size_line to length 2 when a single value is provided prevents this error and improves usability.

  # handle alias
  dots <- list(...)
  if (!is.null(dots[["linewidth"]])) {
    size_line <- dots[["linewidth"]]
  }
  if (length(size_line) == 1) {
    size_line <- rep(size_line, length.out = 2)
  }