Each vertex means 100% of one component; lines parallel to the opposite side show its share.
A triangular graph represents the composition of a whole using three categories. The shares must use the same denominator, be mutually exclusive and add to 100%. Our example classifies one main journey mode per resident as walking, public transport or private transport.
At the walking vertex, walking is 100% and the other two modes are zero. On the opposite side, walking is zero. A line parallel to that side marks a constant walking share. Apply the same rule from each of the other vertices; read the labelled direction rather than assuming all scales increase clockwise.
To plot A, use 30% walking and 50% public transport. Their constant-share lines intersect at one point. Check the third share: 20% private transport. Only two values are independent because the three must sum to 100%.
Movement towards a vertex means that component's share rises. Moving along a line parallel to its opposite side keeps that component unchanged. A percentage shift does not reveal a change in absolute numbers without knowing the population totals.
Step by step
Check the whole
Ensure all three categories are exclusive parts of the same total.
Follow parallel lines
Locate two shares using lines parallel to the opposite sides.
Check the third
Confirm the intersection gives the remaining percentage.
Worked example: Check before plotting
Shares of 40%, 35% and 25% can form one point. Shares of 40%, 50% and 30% cannot: they total 120%. Check overlapping categories or a mismatched denominator instead of forcing the values onto the triangle.
Watch out for this
The triangle can compare any three independent percentages.
It represents three parts of one whole. Percentages from unrelated questions may not sum to 100% and do not belong on this graph.
Check your understanding
Walking is 25% and public transport 45%. What is private transport if these are the only three main modes?
- 70%.
- 20%.
- 30%.