Refactor hardcoded "magic numbers" in legend.grob

[dan-knight]

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Closes #8

The values mentioned in the issue are explained in this comment (found earlier in the file).

# Create a layout for the legends
# Each legend gets an extra row for its title
# There is also an extra empty row/column inserted between each of the legends to allow for nicer spacing

Instead of creating multiple comments to explain the same value, it made more sense to change the repeated hardcoded values to variables.

Testing Results

legends <- list(
	legend = list(
		colours = c('orange', 'chartreuse4', 'darkorchid4'),
		labels = c('Group 1', 'Group 2', 'Group 3'),
		title = expression(underline('Legend #1'))
		),
	# Use dots instead of rectangles
	point = list(
		colours = c('firebrick3', 'lightgrey'),
		labels = c('A label', 'A longer label'),
		# Set dot size
		cex = 1.5
		)
	)
	
legend.grob(
	legends = legends,
	border = list(col = 'blue', lwd = 2, lty = 3),
	border.padding = 1.5,
	between.row = 3
	);