Data Visualization: The Graph

Before we start on the adventure of creating different visualizations, let’s continue to talk about forms of visualization.  Last time we talked about the Table, now let’s take a similar approach and discuss the Graph or the Chart.

Why use a Graph / Chart?

First let me note that I will more than likely use the words graph and chart interchangeably.

  • When you want to communicate a trend or a pattern in your data
  • Graphs/Charts are more interesting than a table of numbers – let’s be honest here
  • Your reader will tend to remember a graph / chart more readily than a table

Types of Charts

I have been asked on many an occasion, what type of chart should I use to represent my data?  There are so many different types of charts, and which one you choose will always depend on what you are trying to communicate to your reader.  Here is a partial list of the different types of charts:

  • Line chart
  • Bar chart
  • Histogram
  • Scatterplot
  • Pie chart
  • Bubble scatterplot
  • Heat chart
  • Area chart
  • Box and whisker chart
  • Radar chart
  • …..

Anatomy of a Chart

Similar to a table, a chart should be able to stand on its own.  A Title, legend, and footnotes, should provide the reader with enough information that they be able to interpret the graph as it was meant to be.  There are many documents available in textbooks and online to provide you with a guideline on the proper construction of a chart, but I will highlight one.

However, let’s first start with the title:

  • It should be clear and concise
  • Also known as the HEADING – according to the Operations Manual for the Canadian Journal of Plant Science, Canadian Journal of Soil Science, and the Canadian Journal of Animal Science.
  • Capitalize the heading in sentence format with no period at the end
  • Do not indent the second and any subsequent lines
  • No units of measure in the title

From the Designing Science book http://dx.doi.org/10.1016/B978-0-12-385969-3.00008-8   you can find a wonderful diagram on the Anatomy of the Chart in Figure 1.  Highlighting the structure of a chart which may include the following items:

  • y-axis
  • y-axis label
  • x-axis
  • x-axis label
  • key to symbols used in the chart
  • Statistical symbols
  • Major ticks
  • Minor ticks
  • Error bars
  • Symbols on the chart

Depending on the type of chart you will be creating all or some of the above features will be extremely important.

Best Practices for creating a Chart

  • 2D chart is always better and easier to understand than a 3D chart
  • Background colour – keep it simple – white works great or the colour of your presentation
  • Axes colour – Use the highest contrast colour – black?
  • Data colour – keep group colours consistent – distinct from each other
  • Gridlines – only if you need them – they can be very distracting
  • Font – use sans serif type fonts.  Helvetica is recommended
  • Significance – use *, **, *** – consistent
  • Error/variability – do not clutter your chart
  • Ticks on your scales – use a natural count

Selecting a Graph or a Table?

When should you use which one?

  • Remember a table is appealing to the “reader”
  • If you need your audience to remember a number or if you need to highlight a number/value – a table may be best
  • Graphs / charts – may have that impact that tables do not.
  • Each has their place and purpose

Examples of Charts

While reviewing these examples of different types of charts published in journals and on the web – think about the best practices listed above.  Do all of these examples follow them?  Are they easily read?  Do they stand alone?

When you embark on creating your own table and/or chart, think about these guidelines and best practices.

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