Taking time with visualisation
Stacked area chart
What it shows
A 100% stacked area chart shows how the constituent parts of a whole have changed over time. The y axis scale is always 100%. Each area of colour represents one part of the whole. The parts are stacked up, usually vertically. The height of each coloured stack represents the percentage proportion of that category at a given point in time. A stacked area chart might be used to show the breakdown of support for different political parties over time.
How to read it
Focus on the colours and assess which chunks of colour are growing or shrinking as they move along the time axis. Check which colour areas take up the largest and smallest slices of the overall total.
Things to beware
You may be drawn to the upward and downward slopes of the coloured area, but this can be misleading. The shapes of the middle series – those not attached to a bottom or top baseline – will be influenced by the values above and below, as well their own.
Note that other forms of area charts show the growing or shrinking of the absolute total rather than the part of a whole. In these cases the y-axis will not be based on a 100% scale.