Visualizing the History of Epidemics

I really like National Geographic. Their magazine is great, their television documentaries are done well and they helped give me a lifelong love of maps. They generate very good information and help shed light on the world we all share. So why is this graphic so awful?

Let's have a look:
National Geographic image

We'll start off by saying that no one will mistake me for Edward Tufte or Stephen Few or Nathan Yau, though I love their stuff, have read it and have tried to adopt as many of their more sensible recommendations as I can. That understood, I think I'm on solid footing when I say that at a minimum, all graphical elements should fit within the display surface. The first three quantities are so massive, that they can't be contained. How big are they? Well, we have the numbers within the circles, but beyond that, who knows? The plague of Justinian looks like it could be Jupiter to the Black Plague's Saturn, with modern epidemics having more of an Earthly size.

Speaking of circles, I try to avoid them. If those three aforementioned experts have taught me anything it's that the human brain cannot easily process the area of a round object. Quick: without looking at the numbers, tell me what's the relativity between HIV and ebola.

Did you have to scroll to look at both objects? I did. Not only do the largest epidemics spill over the display area, they make it difficult to view a large number of data points at the same time. As we scroll down, we eventually land on a display which has Asian flu at the top and the great plague of London at the bottom. Justinian, the black death and medieval history are erased from our thoughts.

And what's with the x-axis? The circles move from one side to the other, but this dimension conveys no meaning whatsoever.

As an aside, although I love having the years shown, it would have been good to use that to augment the graphic with something that conveys how epidemics have changed over time. Population has changed, medicine has changed and the character of human disease has changed. As I look at the graphic, what I tend to extrapolate from this is that surely the plague of Justinian wiped out most of southern Europe, Anatolia and Mesopotamia. In contrast, SARS likely appeared during a slow news cycle.

It would be disingenuous of me to criticize a display without proposing one of my own. So, here goes.

dfEpidemic = data.frame(Outbreak = c("Plague of Justinian", "Black Plague"
                                     , "HIV/AIDS", "1918 Flu", "Modern Plague"
                                     , "Asian Flu", "6th Cholera Pandemic"
                                     , "Russian Flu", "Hong Kong Flut"
                                     , "5th Cholera Pandemic", "4th Cholera Pandemic"
                                     , "7th Cholera Pandemic", "Swine Flu"
                                     , "2nd Cholera Pandemic", "First Cholera Pandemic"
                                     , "Great Plague of London", "Typhus Epidemic of 1847"
                                     , "Haiti Cholera Epidemic", "Ebola"
                                     , "Congo Measles Epidemic", "West African Meningitis"
                                     , "SARS")
                        , Count = c(100000000, 50000000, 39000000, 20000000
                                    , 10000000, 2000000, 1500000, 1000000
                                    , 1000000, 981899, 704596, 570000, 284000
                                    , 200000, 110000, 100000, 20000, 6631
                                    , 4877, 4555, 1210, 774)
                        , FirstYear = c(541, 1346, 1960, 1918, 1894, 1957, 1899, 1889
                                        , 1968, 1881, 1863, 1961, 2009, 1829, 1817
                                        , 1665, 1847, 2011, 2014, 2011, 2009, 2002))
dfEpidemic$Outbreak = factor(dfEpidemic$Outbreak
                             , levels=dfEpidemic$Outbreak[order(dfEpidemic$FirstYear
                                                                , decreasing=TRUE)])
plt = ggplot(data = dfEpidemic, aes(x=Outbreak, y=Count)) + geom_bar(stat="identity") + coord_flip()
plt = plt + scale_y_continuous(labels=comma)

plot of chunk GetDataFrame

I'm showing that data as a bar chart, so everything fits within the display and the relative size is easy to recognize. I also order the bars by starting year so that we can convey an additional item of information. Are diseases getting more extreme? Nope. Quite the reverse. 1918 flu and HIV have been significant health issues, but they pale in comparison to the plague of Justinian or the Black Death. HIV is significant, but we've been living with that disease for longer than I've been alive. If we want to convey a fourth dimension, we could shade the bars based on the length of the disease.

dfEpidemic$LastYear = c(542, 1350, 2014, 1920, 1903, 1958, 1923, 1890, 1969, 1896, 1879
                        , 2014, 2009, 1849, 1823, 1666, 1847, 2014, 2014, 2014, 2010, 2003)
dfEpidemic$Duration = with(dfEpidemic, LastYear - FirstYear + 1)
dfEpidemic$Rate = with(dfEpidemic, Count / Duration)

plt = ggplot(data = dfEpidemic, aes(x=Outbreak, y=Count, fill=Rate)) + geom_bar(stat="identity")
plt = plt + coord_flip() + scale_y_continuous(labels=comma)

plot of chunk AddDuration

The plague of Justinian dwarfs everything. We'll have one last look with this observation removed. I'll also take out the Black Death so that we're a bit more focused on modern epidemics.

dfEpidemic2 = dfEpidemic[-(1:2), ]
plt = ggplot(data = dfEpidemic2, aes(x=Outbreak, y=Count, fill=Rate)) + geom_bar(stat="identity")
plt = plt + coord_flip() + scale_y_continuous(labels=comma)

plot of chunk SansJustinian

HIV/AIDS now stands out as having the most victims, though the 1918 flu pandemic caused people to succomb more quickly.

These bar charts are hardly the last word in data visualization. Still, I think they convey more information, more objectively than the National Geographic's exhibit. I'd love to see further comments and refinements.

Session info:

## R version 3.1.1 (2014-07-10)
## Platform: x86_64-pc-linux-gnu (64-bit)
## locale:
##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
##  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
## other attached packages:
## [1] knitr_1.6        RWordPress_0.2-3 scales_0.2.4     ggplot2_1.0.0   
## loaded via a namespace (and not attached):
##  [1] colorspace_1.2-4 digest_0.6.4     evaluate_0.5.5   formatR_0.10    
##  [5] grid_3.1.1       gtable_0.1.2     htmltools_0.2.4  labeling_0.2    
##  [9] MASS_7.3-34      munsell_0.4.2    plyr_1.8.1       proto_0.3-10    
## [13] Rcpp_0.11.2      RCurl_1.95-4.1   reshape2_1.4     rmarkdown_0.2.50
## [17] stringr_0.6.2    tools_3.1.1      XML_3.98-1.1     XMLRPC_0.3-0    
## [21] yaml_2.1.13
Posted in R

4 thoughts on “Visualizing the History of Epidemics

  1. I’d also suggest using a log scale on the x-axis when the range of data is so large that a few of the classes dominate the graph!

  2. ordering by year definitely makes sense, i would suggest making the bar widths proportional to the duration of the epidemic, so the bar lengths would represent the yearly death count (what you call rate). and the area the total death count. I think that is easier to interpret than the colour scale.

    also – it would take a bit of extra data sourcing – but death rates would be a lot more informative than counts given that the total population has increased substantially over the time covered. only then could you truly tell the story of how epidemics have changed over time.

  3. Absolute numbers can be a bit tricky. This way you have absolute no reference in regard to the size of the issue. I would suggest putting it as a percentage of the world population. It would pose a problem for something like AIDS having a date range but without reference these numbers say very little.

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