When an area chart is the right choice, when another chart reads better, and how to draw one that tells the truth. The same rules hold in every chart library. For picking between chart types in general, see Choosing a chart.
What it is for
An area chart shows how an amount changes over an ordered range, usually time. It is a line chart with the space below the line filled, so the size of the amount reads as well as its trend. Stacked, it shows how a total and its parts change together.
When to use
The trend matters and so does the volume under it, such as sign-ups per month or traffic per day.
A total changes over time and you want to show the two or three parts it is made of. Stack the areas.
The shares of a total change over time and the totals do not matter. Stack the areas to 100%.
You show a range around a value, such as a forecast band or a daily low and high. Fill between a lower and an upper edge.
Use something else when
You compare several series that do not add up to a total. Use a line chart, where every series has its own line and none hides another.
The reader must compare the middle layers of a stack. Use a line chart or one small chart per series, so each starts from the same baseline.
There are only a few points in time, or the categories have no order. Use a bar chart.
Zero is not a meaningful start, such as a temperature or a stock price. Use a line chart, which may start above zero.
You show one share of a total at one moment. Use a pie chart or a single stacked bar.
How it reads
The top edge reads like a line: its position shows the value and its slope shows the change. The filled area adds weight, so the eye reads it as an amount from the baseline. In a stack, only the bottom layer and the total sit on a shared baseline. The layers in between have to be read as the gap between two moving edges, which people judge poorly.
Do it well
Start the value axis at zero. The fill stands for the amount from the baseline, so a cut axis makes the area lie.
Keep the fill light and give the top edge a stroke, so the trend stays easy to follow.
Put the steadiest or most important series at the bottom of a stack, where it keeps a flat baseline.
Keep a stack to about four series. Group the small ones as "Other".
Pick the curve by what the data means. Straight lines for measured points, steps for a value that holds until it changes, such as a price or a plan.
Show where data is missing as a gap, not as a drop to zero. The Missing data guideline explains how.
Pitfalls
Overlapping areas that are not stacked. The one in front hides the ones behind. Stack them, or use lines.
A smooth curve that overshoots. Some curves bend above the highest point or below zero between two points. Use a monotone curve, or straight lines.
A stack of series that are not parts of one total. Adding them up means nothing, so the top edge means nothing. Use lines.
Strong, opaque fills in every layer. The chart turns into blocks of color. Lower the fill opacity and keep the edges.
A legend far from the chart for a stack with many layers. Label the layers directly, or keep the legend in the stack's order.
Variants
Single: one series, filled down to zero.
Stacked: parts of a total, with the top edge as the total.
100% stacked: shares of a total, every point stretched to full height.
Step: a value that holds between changes.
Range or band: a filled interval between a lower and an upper value, such as a forecast range.
Streamgraph: layers stacked around a moving center line, for the overall shape of many series rather than their values.
Compared with similar charts
Against a line chart: the same trend with more weight. Use lines for several series that do not add up, and areas when the amount itself matters.
Against stacked bars: the same parts of a total, for many points in time. With fewer than about eight periods, stacked bars read better.
Against a pie chart: a 100% stacked area shows how the shares change. A pie shows them at one moment only.