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Data Visualization

Bar Chart

: bar chart showing female and male occupations from the 1881 census of England and Wales.

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? 

Histogram Chart

Histogram showing the frequency of specific heights of cherry trees in feet.

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?

Pie Chart

Pie chart showing 4 unlabelled categories.
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? 

Line Chart

 Line chart showing interest rates by compounding frequency

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

Best used for: 

  • When 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? 

Scatter Plot

Scatter plot showing the correlation between oppose and support votes

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? 

Heat Map

Heat map showing high and low percentages in individual cells.

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? 

Maps - Choropleth

Choropleth map of the United States showing 6 categories of vehicles per 1000 people.

Figure 10: Example of a choropleth map.
Bramo126. (2017). Map of each state's vehicles per 1000 people in 2015 
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 numeric variable or one categorical variable

Strength: 

  • Easy to read and create
  • Data points do not overlap
  • Recognizable shape of geographic space does not require labels

Best practices: 

  • Be aware of how region size may confuse message (ie: elections - vast northern regions vs condensed cities)
  • Assess if spatial data is better displayed via graphs

How do I create a choropleth map?

Maps - Symbol

Map of Africa showing proportional dots for fertility and colour coded countries for GDP in US dollars.

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? 

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