Build population pyramids, the back-to-back bar charts of a population by age group and sex, as layers of a 'ggplot2' plot. A position adjustment mirrors one side and stacks subgroups within each side, so bars, labels and lines from any layer land on the correct side while the plot can still be faceted, themed and extended as usual. A coordinate system labels both sides with absolute values, gives them one shared scale, and places the age axis on either side or in a gap at the centre. Further layers count individual records or sum aggregated values, show proportions, label the bars, and draw a second population as a stepped outline for comparison.