plot_intervals_unif: Plot intervals as uniform (box) areas
In jimvine/confinterpret: Descriptive Interpretations of Confidence Intervals

The current implementation of this function uses boxplot to draw its boxes.

plot_intervals_unif(intervals, estimates = NULL,
  interval_value_labels = FALSE, estimate_value_labels = FALSE,
  interval_labels_offset = c(0, 0, box_halfheight + 0.1, box_halfheight +
  0.1), estimate_labels_offset = c(0, box_halfheight + 0.1),
  plot_estimate_marks = FALSE, estimate_mark_points = c(1.2 *
  graphics::strheight("M"), 0.05, -1.2 * graphics::strheight("M"), -0.05), ...)

`intervals`	The interval(s) to be plotted. Two column matrix.
`estimates`	Estimates for each of the intervals (optional).
`interval_value_labels`	Logical value specifying whether interval value labels are to be added.
`estimate_value_labels`	Logical value specifying whether estimate value labels are to be added.
`interval_labels_offset`	Amount to offset interval labels by from the centre of the end of the interval's plot. `c(x1, x2, y1, y2)`.
`estimate_labels_offset`	Amount to offset estimate labels by. `c(x, y)`. Normally want the estimate to be x-located at its value, but may want a y-offset to move it above or below the plot shape that represents the interval.
`plot_estimate_marks`	Whether to plot marks at the x location of the estimates.
`estimate_mark_points`	y positions of the ends of the estimate marks as a numeric vector of length 4. Values are, in order: start (relative to centre), end (relative to box top), start (relative to centre), end (relative to box bottom).
`...`	Further parameters to be passed on.

The default value for the estimate_labels_offset parameter is defined in terms of a variable, box_halfheight. Because boxplot, the underlying plotting function, draws boxes different heights depending on the number of boxes drawn, this is set within the function. For one box the box_halfheight is 0.2; otherwise it is 0.4.