1. CSc4730/6730 Scientific Visualization
Lecture 05
Basic Visualization Techniques
Ying Zhu

2. Graphical Forms Bar chart Time series Table Pie chart Graph
Unconventional chart

3. Bar graphs Used for compare magnitudes among several categories
the evolution in time (the change) of a particular magnitude. 
the comparison of the evolution in time of several categories
Example:

4. Data types Data can be classified into three types:
Categorical or Nominal
The order of the categories is arbitrary
E.g. birds, mammals, tall, small, green or blue…
Ordinal or Sequential
Categorical data with logical ordering
E.g. the months of the year, the alphabet…
Numerical or Quantitative
The data variables mainly used in bar graphs are those of categorical and ordinal types
Bar graph is not very suitable for representing quantitative data.

5. Components
Quantitative axis, Categorical or ordinal axis, and a set of rectangles

6. Main types of bar graphs
Simple
Contains only one data series
Grouped
Contains several data series

7. Main types of bar graphs
Stacked

8. Main types of bar graphs
Floating, bi-directional or paired
Pictorial
Range

9. Create effective bar graphs
Consistency
The same scale, color, or texture should mean the same across the representation

10. Create effective bar graphs
In stacked bar charts, place in the lower part the values that have a lower variation

11. Create effective bar graphs
Use bar graphs to compare
Bar graphs are suitable for a categorical axis
Don’t use bar graph if you have a temporal axis and you would like to see the trends
A line chart can show the trend in a better way.
A line better illustrates the continuity of its values.
Bar graph is appropriate if you have a temporal axis and you are interested in comparing specific values
Bars are very useful to allow the user to compare particular values.

12. Create effective bar graphs
Vertical or horizontal bars?
The properties of the visual pattern shouldn’t be in contradiction with the real properties of what is being represented
E.g. heights are better represented with vertical bars than with horizontal ones, while lengths are better shown with horizontal ones.

13. Create effective bar graphs
When in doubt use vertical bars
Most cultures recognize height as a bigger value, while this does not apply for horizontal extension.

14. Create effective bar graphs
Use paired or bi-directional bar graphs to show differentiated trends amongst levels of a single variable.
Don’t use paired graphs for more than two independent variables
Use stacked bar charts to represent components of a whole in a categorical scale.

15. Create effective bar graphs

16. Case study

17. Case study
Source: gradeinflation.com

18. Case study
Source: gradeinflation.com

19. Visualize time series data
Maximizing data density
Maximizing data-ink ratio
Do not show data out of context
Avoid chartjunk
Use a consistent scale or interval
Be careful when using multiple dimensional objects to represent one dimensional data
Be careful with data aggregation

20. Case study
Keele and Luke, Social Capital and the Dynamics of Trust in Government, American Journal of Political Science, 2007, 51:2, pp

21. Case study
Lebo, et al. Strategic Party Government: Party Influence in Congress, American Journal of Political Science, July 2007, 51:3, pp

22. Case study

23. Case study

24. Tabular Presentation Table is for communication, not data storage
Three rules for tabular presentation
Order the rows and columns in a way that makes sense
Don’t simply go with the alphabetic orders
Round numbers for easier comparison
All is different and important
Summaries or rows and columns are important

25. When to use graphics?
Use sentence for relations between 2 or 3 data at most
Table is suitable for between 3 and 20 data
Graphics is good from 2 on, and it’s especially useful for large amounts of data

26. Pie chart Pie graphs are used to show percentages and proportions
The number of items compared should be kept to five or fewer
Order the segments from large to small, beginning the largest segment at 12 o’clock
A good way to highlight differences is to shade the segments from light to dark
Use patterns or lines and dots to shade the segments (?) (source: Manual of the American Psychological Ass.)

27. Case study
Lessons learned
Bar chart is often better than pie chart

28. Graph
A graph is an abstract object consisting of vertices (or nodes) and edges (or arcs) between pairs of vertices.

29. Rules for graph
Basic rules

30. Rules for graph
Semantic rules

31. Rules for graph
Structure rules

32. Rules for graph
Structure rules

33. Summary of principles of visualization
Maximize data density
Maximize data/ink ratio
The magnitudes must correspond with their representation
Minimize the lie factor
Don’t quote data out of context
Don’t change scale in mid-axis
Encourage comparison

34. Summary of principles of visualization
Order your data in a way that makes the most sense
Clearly label your data
Avoid chartjunk