Optimize boxplot right margin

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After rotating boxplot x-axis labels 45 degrees, as suggested by @dunnpa, some right side labels are truncated by default right margin. To include all x-axis labels in the plot, the right margin is set with the following procedure. However, this procedure needs to be optimized for the following two purposes:

• Guarantee that all x-axis labels are included and not truncated.
• Guarantee that right margin always extend the rightmost label by 5.5 points, which is the default ggplot2 margin.

OpenPedCan-api/src/get_gene_tpm_boxplot.R

Lines 75 to 87 in b265d7b

	# The x-axis labels are long and rotated 45 degrees, so they are out of the
	# plot in the default margin. Increase right margin to fit all text.
	plot_margin <- ggplot2::theme_get()$plot.margin
	if (!identical(length(plot_margin), 4L)) {
	plot_margin <- rep(grid::unit(x = 5.5, units = "points"), 4)
	}
	rightmost_x_label <- dplyr::last(
	levels(gene_tpm_boxplot_tbl$x_labels), default = "")
	# increase right margin by the width of the last x label * 0.71
	plot_margin[2] <- grid::unit(
	x = 0.8, units = "strwidth", data = rightmost_x_label)

Example boxplot with truncated x-axis labels:

library(ggplot2)
data(diamonds)
diamonds$cut <- paste("Very very very long x-axis labels like our boxplot",as.character(diamonds$cut))
qplot(cut,carat,data=diamonds,geom="boxplot") +
  theme(axis.text.x = element_text(angle = -45, vjust = 1, hjust = 0))