Building number confidence: Graphical data

4. Charts and graphs

4.4. Area chart

An area chart is similar to a line chart, but highlights proportions by using colours, or shading, to fill the areas below the lines.

It can be difficult to discern individual values, however, particularly where areas overlap.

An area chart with some areas only partially visible as others are overlapping them.

Data from: .

  Americano Latte Flat white Cappuccino Tea
Monday 76 14 20 12 35
Tuesday 60 30 14 25 17
Wednesday 57 28 30 22 25
Thursday 37 32 12 27 14
Friday 70 36 18 32 28
Saturday 72 35 27 16 32
Sunday 55 25 23 27 40

A stacked area chart reorders the categories to clearly show the proportion of each to the total value. However this reordering can cause confusion. For example the highest sales (Americano) in our Daily sales report appear at the bottom of the stack, with the lowest (teas) at the top of the stack, which may not seem logical.

A stacked area chart separates and displays the categories as parts of a whole.

As with line charts, a simple area chart can be most useful when used to show relationships, and highlight differences, between one or two data sets

Looking at teas and Americanos only, from the Daily sales report, it is clear that sales of Americanos are at least double those of teas on most days.

Simplified area chart showing two data sets: Americano and tea sales.

 

 Data check

Looking at the chart above, on what day of the week do the number of teas sold reach a value greater than half of the number of Americanos sold?

On Sunday, the proportion of teas sold is clearly greater than half of the Americano sales. Americano sales are between 50 and 60 on the y axis, and teas are around 40.