Data Visualization

Figure 4: Example of a bar chart.
Smith, V. (1996). Male and Female Occupations From the 1881 Census of England and Wales.
CC BY-SA 4.0

Best used for:

• Reading numbers accurately from the chart
• Comparing individual bars to each other
• Exploring overall trends across categories
• Note, see histograms for continuous data.;

Data

• One (or more) text (categorical) variable
• One numerical variable

Strength

• Familiar to most audiences
• High perceptual accuracy because they use alignment and length

Best practices

• Use horizontal bars to enhance label readability
• Consider sorting bars by length when alphabetical order is not important
• Axis must start at zero

How do I create a bar chart?

Figure 5: Example of a histogram.
Unknown. (2008). Heights of Black Cherry Trees. 
CC BY-SA 3.0

Best used for

• To show the density and distribution of continuous numerical data.  Note, that bar charts plots categorical data

Data

• One continuous numerical variable

Strength

• High perceptual accuracy because they use alignment and length

Best practices

• Divide the entire range of values into a series of intervals (bins) and then count how many values fall into each interval. 
• The bins are usually specified as consecutive, non-overlapping intervals of a variable.
• The bins (intervals) must be adjacent, and are often (but are not required to be) of equal size 

How do I create a bar chart?

Figure 6: Example of a pie chart.
Sylvanmoon. (2016). Pie-Chart.
CC0

Best used for

• When separate categories add up to a meaningful whole

Data

• One text (categorical) variable
• One numerical variable

Strength

• Familiar to most audiences

Best practices

Pie charts are very difficult for humans to interpret accurately due to angle and area.  To improve perception:

• Categories MUST add up to meaningful whole
• Only use when significant differences exist in category size
• Sort by size
• Ideally, limit your categories to two: the data point you wish to highlight and everything else.
• Avoid 3D versions as they cause distortion.

How do I create a pie chart?

Figure 7: Example of a line chart
Kazov. (2015). Compound Interest (English).
CC BY-SA 4.0

Best used for

• Visualizing continuous and temporal data

Data

• One date (categorical) variable on the x-axis (horizontal)
• One or more numerical variable on the y-axis (vertical)

Strength

• Familiar to most audiences
• Clearer choice over grouped and stacked bar charts when multiple categories are needed

Best practices

• Use colour to differentiate categories
• Too many lines creates difficulty in readability
• Place labels to improve readability
• Add small dots to clarify measurement locations
• Axis must start at zero

How do I create a line chart?

Figure 8: Caption: Example of a scatter plot.
Opabinia regalis. (2015). Scatter Plot of FfA votes on English Wikipedia.​
CC BY-SA 4.0

Best used for

• Showing how one variable affects the other, or for showing precise, data dense visualizations, correlations, and clusters between two variables.

Data

• Two numerical variables  
• When a third numerical variable can be added to create a bubble chart.  The third variable changes the size of the dots. 

Strengths

• High perceptual accuracy
• Visual representation of relationship between variables including:
• Show pos / neg correlation, clusters, outliers, 

Best practices

• Scatter plots are more commonly used in the social and hard sciences.  It may be necessary to provide training for your audience for accurate interpretation.
• Use transparency or aggregation for stacked dots
• Show variables requiring most precision on axis, 3rd variable to encode size

How do I create a scatter plot?

Figure 9: Example of a heat map.
LuckyDawg10. (2017). Mike Zunino Swing% vs All Pitchers.
CC BY-SA 4.0

Best used for

• Showing general trends

Data

• Data grid of two numerical or categorical variables
• Third variable is (often the number of data points associated with the particular row and column) is encoded as the colour of the cell.

Strength

• Useful for representing large amounts of data
• Space efficient

Best practices

• Heat maps are not commonly used making it difficult for most people to read.  Provide training for your audience for accurate interpretation.
• Do not use if numerical value needs to be read precisely
• Use colour encoding to give a sense of high and low values
• Heat maps are highly dependent on the order of the rows and columns to reveal similarities between different categories within the same variable

How do I create a heat map?

Figure 11: Example of a symbol map.
GraceN.Cartography. (2016). Adolescent Fertility and Poverty in Africa.jpg
CC BY-SA 4.0

Best used for

• When spatial patterns are important

Data

• Place variable (x,y; longitude and latitude; address; place name)
• One or two numeric variable or categorical variable

Strength

• Easy to read and create
• Can encode two separate variables in addition to location
• Best used for cities

Best practices

• Use transparency or aggregation for overlapping circles
• If precision is important, use bar charts instead

How do I create a symbol map?