Party Seat Shares Across Elections

A single election result is a snapshot. It tells you who won and by how much, but nothing about direction. Line up seat shares across six or eight elections in a row and a snapshot becomes a trajectory: a once-dominant party sliding from 60% of seats to 35% over twenty years, or a new challenger climbing from zero to a plurality. Patterns that are invisible in one result, realignment, decay, sudden collapse after a scandal, become obvious once you see the whole sequence.

A stacked area chart is the usual tool for this. Time runs along the bottom, seat share from 0 to 100% runs up the side, and each party gets a colored band stacked on top of the others so the bands always sum to the full legislature. Reading one is just tracking the thickness of a single party's band as it moves left to right: a widening band means the party is winning more seats over time, a narrowing one means decline. Where two parties' bands cross is where control changed hands.

The natural extension is plotting vote share and seat share on the same axes instead of seat share alone. The two lines rarely match. A party might win 35% of votes but 55% of seats under first-past-the-post, because winner-take-all districts amplify a plurality into a majority. The size of the gap between the lines, at every single election, is a direct measure of how much the electoral system itself, not voters, is manufacturing or punishing a party's power. This connects straight back to why proportional and majoritarian systems produce such different legislatures from similar votes.