Parliament Chart
A hemicycle of coloured dots representing individual seats — the iconic way to show political seat distribution, coalition sizes, and majority thresholds at a glance.
// 01 — The chart
What it looks like
A parliament chart showing 80 seats distributed across four parties. Seats are arranged in concentric semicircular rows, coloured by party affiliation.
// 02 — Definition
What is a parliament chart?
A parliament chart (also called a hemicycle diagram) is a semicircular arrangement of individual dots, each representing one seat in a legislative body. Dots are coloured by party or faction to show the distribution of political power.
The layout mimics the physical seating of many real-world parliaments — parties sit together in arcs, with the ruling coalition typically on one side and the opposition on the other. This makes the chart immediately intuitive for anyone familiar with democratic institutions.
Beyond politics, parliament charts can represent any scenario where individual units need to be grouped and counted — board votes, jury composition, or committee membership.
Fun fact: The parliament chart became a staple of election coverage thanks to Wikipedia, which maintains hemicycle diagrams for virtually every national legislature. The open-source tool “parliament-svg” and similar libraries made generation easy.
// 03 — Anatomy
Parts of a parliament chart
// 04 — Usage
When to use it — and when not to
- Showing seat distribution in a legislature or council
- You want the audience to count individual units (seats, votes)
- Visualizing coalition possibilities — which parties sum to a majority
- The data maps naturally to a hemicycle (legislative bodies, committees)
- Creating engaging election coverage infographics
- Data isn't about discrete, countable units
- You have hundreds of seats — dots become too small to distinguish
- Precise percentage comparison is the goal — use a bar chart
- The metaphor of "seating" doesn't apply to your context
- You need to show change over time — use paired charts instead
// 05 — Reading guide
How to read a parliament chart
Check the legend
Identify which colour represents which party or faction. This mapping is essential to reading the chart.
Spot the largest block
The party with the most dots has the most seats. Visually, the largest contiguous colour cluster is the dominant party.
Find the majority line
The total seat count is often shown in the center. Half of that + 1 is the majority threshold. Does any single party reach it?
Assess coalition potential
Can two or more party clusters add up to cross the majority threshold? Adjacent colours often represent potential coalition partners.
Compare election results
If shown alongside a previous parliament chart, compare the size of each colour block to see which parties gained or lost seats.
// 06 — Data format
What data do you need?
A list of parties with their seat counts and colours. The total seats determine how many dots to draw.
[
{ party: "Party A", seats: 120, color: "#c94a2e" },
{ party: "Party B", seats: 95, color: "#e8c4b8" },
{ party: "Party C", seats: 55, color: "#f5ede9" },
{ party: "Independent", seats: 30, color: "#f0e4e0" }
]
// Total = 300 seats// 07 — Construction
How to build one
Determine the total seat count and the number of concentric rows needed (typically 3–8 rows).
Distribute seats across rows, with more seats in outer rows (larger circumference = more space).
Position each dot at equal angular intervals along its semicircular row.
Sort parties left-to-right (political spectrum) and assign colours sequentially to dots.
Add a central label showing total seats and a legend mapping colours to party names.
// 08 — Common mistakes
Mistakes to watch out for
Dots too small to see
With 500+ seats, individual dots become indistinguishable. Use fewer rows or group seats into blocks of 5–10.
Random party ordering
Conventionally, parties are ordered left-to-right along the political spectrum. Random ordering confuses the audience.
No majority indicator
The whole point of the parliament chart is to show whether a party or coalition can govern. Always mark the majority threshold.
Using it for non-discrete data
Parliament charts work because each dot = 1 seat. If your data isn't about countable units, use a pie or donut chart.
Missing legend
Without a legend mapping colours to party names and seat counts, the chart is just a semicircle of coloured dots.
// 09 — Real-world examples
Where you’ll see it in the wild
Election coverage
Every major news outlet uses parliament charts for election night results — BBC, CNN, Reuters, The Guardian.
Wikipedia
Nearly every national legislature's Wikipedia page features a parliament diagram showing the current seat distribution.
Political analysis
Think tanks and policy institutes use parliament charts to illustrate coalition scenarios and voting bloc dynamics.
Corporate governance
Board of directors vote distributions, shareholder voting power, and committee composition.
// 10 — Quick reference
Key facts at a glance
Hemicycle diagram, semicircle seat chart
Legislative seat distribution, coalition analysis
50–400 (beyond that, dots become tiny)
Concentric semicircular rows of dots
parliament-svg, D3.js, Datawrapper, Flourish
1 dot = 1 seat
// 11 — Accessibility
Making it accessible
Include a data table listing parties with exact seat counts
Add an aria-label on the SVG summarizing the distribution
Use party patterns or icons in addition to colour for colour-blind accessibility
Ensure the legend includes both colour swatches and numeric seat counts
Provide a text summary: "Party A holds 120 of 300 seats (40%)"
// 12 — Variations
Common variations
Full circle parliament
360° arrangement used for round chambers (e.g. European Parliament). Less common, but shows the same data.
Grouped dots
Dots are clustered by party with gaps between groups, making party boundaries clearer than interleaved seating.
Before/after comparison
Two hemicycles side by side showing election results — one for the old composition, one for the new.
Weighted dots
Each dot represents multiple seats (e.g. 1 dot = 5 seats) for legislatures with 500+ members.
// 13 — FAQs
Frequently asked questions
What is a parliament chart?+
A parliament chart (also called a hemicycle diagram) is a semicircular arrangement of individual dots, each representing one seat in a legislative body. Dots are coloured by party or faction to show the distribution of political power.
When should you use a parliament chart?+
Use a parliament chart when showing seat distribution in a legislature or council. It also works well when you want the audience to count individual units (seats, votes), and when visualizing coalition possibilities — which parties sum to a majority.
When should you avoid a parliament chart?+
Avoid a parliament chart when data isn't about discrete, countable units. It is also a poor fit when you have hundreds of seats — dots become too small to distinguish, or when precise percentage comparison is the goal — use a bar chart.
What data do you need to make a parliament chart?+
A list of parties with their seat counts and colours. The total seats determine how many dots to draw.
How is a parliament chart different from a waffle chart?+
Both a parliament chart and a waffle chart can look similar at first glance, but they answer different questions. Reach for a parliament chart when the comparisons and patterns it was designed to reveal match what you need to communicate, and choose a waffle chart when its particular strengths better fit your data and audience.
What is another name for a parliament chart?+
Parliament Chart is also known as Hemicycle diagram, semicircle seat chart. The name varies between fields, but the visualisation technique is the same.
What size of dataset works best for a parliament chart?+
Parliament Chart works best for Legislative seat distribution, coalition analysis. Outside that range the chart either looks empty or becomes too cluttered to read clearly.
Are parliament charts accessible to screen readers?+
Yes — a parliament chart can be made accessible to screen readers by pairing it with a clear text summary of the key insight, ensuring color choices meet WCAG contrast guidelines, adding descriptive alt text or aria-label to the SVG, and offering the underlying data as an HTML table fallback for assistive technologies.