Choosing the right graph type is an important part of any data visualisation project, with each graph usually following its own set of design rules. But the majority of graph types use only one or two basic design elements to display data. By understanding best practice visualisation of these elements, graphs can always be designed correctly.
Graph type can be simplified.
Many tools exist to help make the right graph choice when visualising data. Most approach the decision from the perspective of the data format. Data with a trend over time could be best visualised in a line graph, where part-to-whole data would be more effective as a bar or pie. Graph type is an important decision but too often the right graph choices are still visualised wrong.
“All graph axes should start at zero”. This is a statement I hear debated a lot. To set the record straight, this is wrong. Not all graph types require their axes to start at zero. But how do you know what rules to follow for what graphs?
Each graph type has best practice design rules, and learning these will enable you to visualise them correctly. But many of these design rules are repeated for multiple graph types because graph type can be broken down and simplified further.
Graphs can be designed from an element level.
The majority of graph types use one or two basic graph elements to visualise data: area and/or point.
These are not to be confused with design principles (such as Gestalt) which will help to add order and reduce graph clutter.
If your graphed data is shaded, you’re using area to visualise it. If your graphed data is plotted, you’re visualising it using point (or position). For some graph types, you’ll have to do both. Understanding graph design at an element level will enable you to correctly create most graphs.
Graph types using only area to display data include Proportional Area, Waffle, and Tree graphs.
Area displays data differences through relative size. When visualising data using area, there must be a zero baseline. Without a zero baseline the area used to represent data is incorrect and deceiving to the viewer.
Visualising Point (or position)
Graph types using only point (or position) to display data include Scatter, Line, and Bump graphs.
Point displays data differences through relative position, rather than size (see area). Because of this, graph axes don’t have to start at zero.
Some graph types connect their points with lines e.g. Line and Connected Scatter graphs. Lines are not graph elements, they provide a connection between points (an application of the Gestalt principle - Connectedness). Cole Nussbaumer Knaflic offers great advice on when to apply this: “the lines that connect the points need to make sense”. Line graphs are good for visualising time series data, and for most people it makes sense to connect the points.
When displaying data using point, all points should be the same size. If they aren’t a uniform size, area is also being used to visualise data (such as a Bubble graph). In the case of the Line graph, points are often smaller than the lines used to connect them.
Visualising both Area and Point
The majority of common graphs combine area and point to display data. These include Bar, Area, Bubble, and Pie graphs.
Graphs with area and point elements compare data using both relative size and position. It’s important to understand where in the graph each element is used, to visualise it correctly.
Area can be measured from horizontal, central, or vertical baselines. Zero baselines will differ between graph types.
Points can be plotted on vertical, horizontal or circular scales. Axes direction will differ between graph types.
Understanding graph element design will improve your data visualisation.
Applying visualisation rules to graph elements will help you design any type of graph correctly.
Bar graphs have an area element so they must have a zero baseline, so as to not distort this element.
Line graphs don’t have an area element (only point), so they don’t have to start at zero because they are visualising relative position.
Bubble graphs have area and point elements. In this case, the area's zero baseline is central to each circle, therefore the point axis doesn’t have to start at zero.
When choosing a graph type, remember: area and point differences are not perceived by people with equal success.
“Our ability to perceive differences in 2-D areas hasn’t evolved to the same level of accuracy as our perception of differences in 2-D position, perhaps because it was more important for survival that our ancestors could detect the exact location of the sabre-toothed tiger, rather than its exact size” – Stephen Frew