Data Visualisation Outline
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Data Visualisation Outline • What is data visualisation • Different types of visualisation • Types of data and ways to encode it • Technologies for visualisation • An intro to D3 Learning Objectives • By the end of this presentation, you will be able to – Describe • What data visualisations can be used for • The difference between exploratory and explanatory visualisations • How different types of data can be encoded visually • Understand the difference between a visualisation library such as D3 and visualisation tools available on the Web • Understand some important concepts about D3 WHAT IS A DATA VISUALISATION? A Basic Definition (1) • “The Visual Display of Quantitative Information” Edward Tufte • The title of the book offers a good definition • The book itself is a classic on the subject • http://www.edwardtufte.com/tufte/books_vdqi A Basic Definition (2) • Or: – “to communicate information clearly and efficiently via statistical graphics, plots and information graphics” – https://en.wikipedia.org/wiki/Data _visualization Detroit - Data • Consider the difference between the display of the same data in the following two slides • Which of these best conveys the story behind the data? • Why is this? Detroit - Data • Available from: http://portal.datadrivendetroit.org/datasets/5efe44008cad4ea98377e7129c98c69c_0 Detroit: Visualising Occupancy https://www.motorcitymapping.org/#t=parcels&s=detroit&f=all&x=preset2 Data Visualisation in the 19th Century • John Snow investigated cases of Cholera in London in 1854 • He observed clusters of cases around Broad Street • Having investigated further, he ascertained that all Cholera cases were from people who drank water from the Broad Street pump • He had the handle removed, and the incidences subsequently reduced • The Visualisation of the Cholera cases is on the next slide. Instances of Cholera in London, 1854 Types of Data Visualisation • Generally split into two categories – Exploratory – Explanatory Exploratory Data Visualisation • Main purpose is to ‘explore’ the data • Allow you to look at data from different angles • Result: – Get a sense of what the data is telling you – Find the interesting nuggets of the data Explanatory Data Visualisation • Main purpose is to ‘explain’ your findings • Focus on communicating the key nuggets to others • A focus on – Context – Story – Elimination of clutter Explanatory Data Visualisation (1) http://wheredoesmymoneygo.org/bubbletree-map.html#/~/total Exploratory Data Visualisation – Visualisations such as these allow you to change the filters to ‘explore’ the data from an angle that is interesting to you http://www.oecdbetterlifeindex.org/ Data for Visualisation • The first step to making a good visualisation is understanding what type of data you have. • Context – what are you trying to display? • Depending on the data, different styles of visualisation may be suitable. Data Types • Continuous – Data for which there is no exact value – E.g., Amount of rain in the UK this summer • Discrete – Typically an integer, a finite, identifiable number – E.g., number of cars owned • Nominal – Categories of data with no order. – E.g., favourite football team Data Encodings • Encoding is mapping data to a visual object • Different data require different encodings • Think about these three types of data, before looking at the next slide. Discrete Continuous Nominal Data with a finite, identifiable Values which cannot be exactly Categories of data with no order number determined E.g., number of cars owned E.g., amount of rain in the UK this E.g., favourite football team summer Retinal Variables (1) SIZE ORIENTATION / ROTATION COLOUR • These are useful for displaying ordered data – Both discrete and continuous data types would work here – E.g. larger objects to represent owning more cars – E.g. darker blue to identify areas on a map where there has been more rainfall Retinal Variables (2) COLOUR (Hue) • These are useful for displaying nominal data – E.g. red to represent Manchester United supporters, blue to represent Chelsea Video • Hans Rosling – 200 Countries, 200 Years, 4 Minutes - The Joy of Stats - BBC Four – https://youtu.be/jbkSRLYSojo • Note down any data encodings you see – For each variable • List what type of data it is • And what visual encoding is used (e.g. size, position, colour…) How Did You Do? Variable Type Encoding Life Expectancy Quantitative (discrete) Position (y) Income Quantitative (continuous) Position (x) Total Population Quantitative (discrete) Size Geographical Region Nominal Colour Time Ordinal Animation Visual Encodings • In 1985, Cleveland and McGill published in the Journal of the American Statistical Association • ‘Graphical Perception and Graphical Methods for Analyzing Scientific Data’ • They proposed basic guidelines for choosing an appropriate graphic form • The paper lists and ranks 10 ‘elementary perceptual tasks’ Cleveland and McGill • Rank of graphic properties based on human ability to understand information • These are particularly relevant for detecting differences and making comparisons – Position along a common scale – Position on identical but nonaligned scales – Length – Angle, slope – Area – Volume, density, colour saturation – Colour hue • For accurate comparisons, graphical forms from the top of this list should be used. Cleveland and McGill - Example It is easier for humans to perceive the differences the values for 3.5 X=0.2 and X=0.4 using position along a common scale (on the scatter plot), than by using area (in the bubble chart). 3 2.5 2 1.5 1 0.5 0 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 Chartjunk • If something can be removed from a chart without changing its meaning, it’s ‘chartjunk’. • Why would one want to combat chartjunk? – Obscures true meaning and story. – Imagery is not information. – Imagery draws attention away from the data • Short-term memory resources are used to identify the images rather than understand the chart. • Term coined by Edward Tufte in the 1980s Chartjunk Example Number of customers 6000 5000 4000 Necessary? 3000 Number of customers 2000 Necessary? 1000 0 Year 1 Year 2 Year 3 Year 4 Year 5 Necessary? The Beauty Paradox • How complex should a graphic be? – How much should you show? • Tufte emphasises minimalism – Communicate as much information from as few pixels as possible • Depending on the audience, this may or may not be the best approach • When creating a visualisation, you must balance the need to focus on the story of the data, with art and design decisions to help your audience engage with and remember the visual. TECHNOLOGIES BEHIND VISUALISATIONS ON THE WEB Web Technologies JavaScript Behaviour CSS Presentation HTML Structure Hypertext • Text that contains links to other texts. • E.g. HTML – Hypertext Markup Language – Defines basic structure and content of a Web page using markup ‘tags’. – Each tag describes a type of content • <body></body> For the body of an html page • <p></p>A paragraph – Read by a Web browser, which decides how to display the content. CSS • Cascading Style Sheets define the design and presentation of an HTML document. • When the Web browser is deciding how to display the content, CSS provides information about the look and feel of each element p { font-family: “Times New Roman”; font-size: 12px; color: blue; } “Make all paragraphs use Times New Roman font, size 12, and colour the text blue.” Javascript • The third essential technology behind the World Wide Web – The ‘programming language of the Web’ – Programs the behaviour of a Web page • Runs on the user’s machine, rather than the Web server (client-side language) • Defines how the page should respond (behave) when there is a user interaction or event – E.g. what should happen when the user clicks on something? Technologies for Visualisation Low Level High Level WebGL D3.js NVD3 (reusable D3 charts) RAW (Web app, paste data) Canvas Dimple.js (D3 for business analytics) Chartio (paid) SVG Rickshaw (D3 for time series) Plotly CartoDB Tableau More Complex, Less Complex, Programmable so Mostly automated so more more confined to templates, customisable, Less Powerful. More Powerful. D3.js (1) • Data-Driven Documents • JavaScript Library • Uses HTML, SVG and CSS to create visualisations from data – Drives the connection between data (provided by user) and documents (rendered by the Web browser) • Aimed at creating explanatory visualisations, rather than exploratory • https://d3js.org/ D3.js (2) • Four general steps: – Loading the data into browser’s memory – Binding the data to elements within the document – Transforming elements by setting visual properties based on the bound data – Transitioning elements between states • Response to user input • Read more in Interactive Data Visualization for the Web by Scott Murray (O’Reilly, 2013) D3.js (3) • Dynamically access the DOM behind a web page to apply styles – DOM: • Document Object Model • Hierarchical Structure of HTML elements • Allows things like – d3.select(“body”).append(“p”).text(“New Paragraph”); D3: Chain Syntax (1) • Using “.”s to chain methods together • Perform several actions in a single line of code • Fast and easy, but can cause debugging problems later • JavaScript doesn’t care about whitespace or linebreaks – d3.select(“body”) .append(“p”) .text(“New Paragraph”); D3: Chain Syntax (2) • d3.select(“body”).append(“p”).text(“New Paragraph”); – First passes ‘select’ a CSS selector – “body”, returning the first element in DOM that matches. – Then creates a new DOM element “p” and appends it to the end of the previously selected element.