[This is a guest post by Severino Ribecca*, as part of a series dedicated to each individual kind of chart that he has read into as part of his main research project.]
The chart we’ll be looking at in this post is Area Graphs, a graph which is essentially a Line Graph with the area below the line filled in.
A Simple Area Graph is drawn by plotting data points on a cartesian coordinate grid, then joining a line between the points and finally filling in the space below the completed line. This give Area Graphs their iconic rocky mountain-like appearance. Area Graphs are ideal for showing trends, by showing the development of quantitive values over intervals or time periods.
The first appearance of a Area Graph or Line Graph was a chart on the national debt of England in the 1786 book The Commercial and Political Atlas, by William Playfair, a Scottish engineer and political economist.
Data with negative numbers can be shown well with Simple Area Graphs:
Curved, Stepped and Segmented Lines
Here are three ways Area Graphs can be generated:
Curved Area Graphs smooth out the lines connected by all the points. Stepped Area Graphs are used to highlight differences and similarities more dramatically then your typically used Segmented Area Graph, which just connects points in a direct straight line.
Multiple Data Series
Simple Area Graphs only include one data series, so the only way to include additional data series is to use a grouped area graph or a stacked area graph.
When Area Graphs are simply “grouped”, all line plots start from the same axis. However, a “stacked” Area Graph has each line-plot start from the point left by the previous line-plot. The diagram below demonstrates this:
An alternative solution to the overlapping problem would be to make each line-plot transparent, so that you can see any part of the Area Graph underneath as a sort of “layered” Area Graph.
Although there’s a limit to with technique and any more then 2-3 data series would start to make the graph difficult to read.
There are also 100% Area Graphs that show a part-to-the whole relationship over intervals:
Finding software that will produce simple, stacked and grouped Area Graphs is extremely easy, as most office software like word processors and spreadsheets have the option to generate charts, including Area Graphs. You can also find a list of online tools that generate Area Graphs on both my reference pages on Area Graphs and on Stacked Area Graphs.
*Severino Ribecca is a British graphic and information designer interested in data visualization. Currently he’s building an online library of different information visualization methods called The Data Visualisation Catalogue. You can follow the project’s updates on Twitter (@dataviz_catalog) and support further developments on the Patreon Page.