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

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

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: http://www.gradeinflation.com

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.

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

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

Main types of bar graphs Stacked

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

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

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

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.

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.

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.

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.

Create effective bar graphs

Case study

Case study Source: gradeinflation.com

Case study Source: gradeinflation.com

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

Case study Keele and Luke, Social Capital and the Dynamics of Trust in Government, American Journal of Political Science, 2007, 51:2, pp. 241-254

Case study Lebo, et al. Strategic Party Government: Party Influence in Congress, 1789-2000. American Journal of Political Science, July 2007, 51:3, pp. 464-481.

Case study

Case study

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

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

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.)

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

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

Rules for graph Basic rules

Rules for graph Semantic rules

Rules for graph Structure rules

Rules for graph Structure rules

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

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