Abuso di Treemap

L’uso di visualizzazioni per sintetizzare in maniera immediata delle informazioni quantitative è uno strumento potente per permettere una facile e rapida comprensione delle informazioni.

Tuttavia quando le visualizzazioni sono utilizzate in maniera casuale, senza cura, il risultato è una nebbia o un’alterazione delle informazioni che si vogliono trasmettere.

Un esempio significativo di quest’ultimo caso è  Quanto spende lo Stato per l’educazione e la cultura? Italia maglia nera in Europa per l’istruzione (7,9% del Pil).

Il mio obiettivo qui è quello di analizzare la visualizzazione e di evidenziare i principali problemi. Questo è anche uno degli esercizi assegnati nel corso di Visualizzazione dell’Informazione Quantitava.

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Annunci

Misusing Bar Graphs

Today I noticed this tweet

The referenced picture is the following one:

Employment

Yet another example of bad visualization. As with other examples I wonder if it is just a case of visual illiteracy or a malicious use of visualization.

The graph has a series of problems:

Due to non-zero base bars, it has a large lie factor (2.2) obtained by comparing:

  • ratio of real values: 87.8 : 61.9
  • ratio on graph: 37.8 : 11.9

The lie factor is due to the non-zero base of the bars.

It uses a dual scale that confuses and makes very hard a visual comparison of the values and further distorting the compared values.

The dual scale is not mentioned anywhere and it is not clear which values refer to which scale.

In general the usage of bars is not the most appropriate visual representation if the goal is to show a trend or evolution in time. Points and lines are a much better choice.

EmploymentRedesign

 

Bar vs Dot Charts (in Excel)

I often observe (on the web or in my students works) bar charts used inappropriately. What often happens is that bar charts are used with a non-zero baseline, which implies telling a visual lie.

Here I report a very simple example of how the issue emerges when drawing a chart in Excel and a possible solution (using a dot chart).

Disclaimer: this is quick post, apologies for typos and lack of clarity.

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Bullet Graph in R

Bullet graphs are an effective and efficient visual representation for key indicators (e.g. KPI), that were proposed by Stephen Few. I find them highly suitable to build dashboards. For this purpose I developed an implementation in R to draw such graphs.

In summary a bullet graph is a variation of a bar graph with additional references that consist of a thick line that represents the reference point (e.g. benchmark, goal, or previous value) and a background that identifies three levels (e.g. Low-Medium-High, Bad-Average-Good, etc.).

The figure below illustrates the main components, though a complete specification is available on Few’s web site.

Bullet Graph elements (from Wikipedia)

The function bulletgraph() provides a simple interface to plot a bullet graph, for instance to reproduce the example above we can use the following statements:

par(mfrow=c(2,1), mar=c(2,9,.1,1))

bulletgraph(x=270,ref=260,limits=c(0,200,250,300),
            name= "Revenue 2005 YTD",subname="(U.S. $ in thousands)",
            colored=F)

bulletgraph(x=270,ref=260,limits=c(0,200,250,300),
            name= "Revenue 2005 YTD",subname="(U.S. $ in thousands)")

Which generates the following diagram:

Bulletgraph

Example of generated bullet graph

The code is available as open-source under the GPL at:
http://softeng.polito.it/software/R/BulletGraph.R

Enjoy and let me know!

Update: added an option (colored=F) to have gray scale background (as recommended in the specification), and a subtitle to be able to reproduce the example.

Update 2: now the revised and tested code, with documentation too, is available on GitHub, here: https://github.com/mtorchiano/MTkR/wiki/Bullet-Graph

Presentazione della tesi di laurea: alcuni consigli

Come ogni sessione di laurea mi ritrovo a rivedere le slide per la presentazione delle tesi che ho seguito come relatore davanti alle commissioni di laurea. E poi ad assistere alle presentazioni.

Provo a riassumere qui alcuni principi e linee guida che di solito fornisco ai miei studenti, come promemoria mio e nella speranza che possa essere di aiuto ad altri.

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