This function is designed to automate the process of producing a Tufte style slopegraph using
I’ve been aware of slopegraphs and bumpcharts for quite some time, and I certainly am aware of Tufte’s work. As an amateur military historian I’ve always loved, for example, his poster depicting Napoleon’s Russian Campaign. So when I saw the article from Murtaza Haider titled “Edward Tufte’s Slopegraphs and political fortunes in Ontario” I just had to take a shot at writing a function.
To make it a little easier to get started with the function I have taken the liberty of providing the cancer data in a format where it is immediately usable. Please see
Long term I’ll try and ensure the version on
CRAN is well maintained but for now you’re better served by grabbing the current version from GITHUB.
If you’re unfamiliar with slopegraphs or just want to see what the display is all about the dataset I’ve provided can get you started in one line
Optionally you can provide important label information through
Caption arguments. You can suppress them all together by setting them
= NULL but since I think they are very important the default is to gently remind you, that you have not provided any information. Let’s provide a title and sub-title but skip the caption.
It’s all well and good to get the little demo to work, but it might be useful for you to understand how to extend it out to data you’re interested in.
You’ll need a dataframe with at least three columns. The function will do some basic error checking and complain if you don’t hit the essentials.
Timesis the column in the dataframe that corresponds to the x axis of the plot and is normally a set of moments in time expressed as either characters, factors or ordered factors (in our case
newcancer$Year. If it is truly time series data (especially with a lot of dates you’re much better off using an R function purpose built for that). In
newcancerit’s an ordered factor, mainly because if we fed the information in as character the sort order would be
Year 10, Year 15, Year 20, Year 5which is very suboptimal. A command like
newcancer$Year <- factor(newcancer$Year,levels = c("Year.5", "Year.10", "Year.15", "Year.20"), labels = c("5 Year","10 Year","15 Year","20 Year"), ordered = TRUE)would be the way to force things they way you want them.
Measurementis the column that has the actual numbers you want to display along the y axis. Frequently that’s a percentage but it could just as easily be any number. Watch out for scaling issues here you’ll want to ensure that its not disparate. In our case
newcancer$Survivalis the percentage of patients surviving at that point in time, so the maximum scale is 0 to 100.
Groupingis what controls how many individual lines are portrayed. Every attempt is made to color them and label them in ways that lead to clarity but eventually you can have too many. In our example case the column is
newcancer$Typefor the type of cancer or location.
This is loosely based off a blog post from Murtaza Haider titled “Edward Tufte’s Slopegraphs and political fortunes in Ontario” that led to my developing this function chronicled here.
In this case we’re going to plot the percent of the vote captured by some Canadian political parties.
The data is loosely based on real data but is not actually accurate.
moredata$Date is the hypothetical polling date as a factor (in this case
character would work equally well).
moredata$Party is the various political parties and
moredata$Pct is the percentage of the vote they are estimated to have.
moredata <- structure(list(Date = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L), .Label = c("11-May-18", "18-May-18", "25-May-18"), class = "factor"), Party = structure(c(5L, 3L, 2L, 1L, 4L, 5L, 3L, 2L, 1L, 4L, 5L, 3L, 2L, 1L, 4L), .Label = c("Green", "Liberal", "NDP", "Others", "PC"), class = "factor"), Pct = c(42.3, 28.4, 22.1, 5.4, 1.8, 41.9, 29.3, 22.3, 5, 1.4, 41.9, 26.8, 26.8, 5, 1.4)), class = "data.frame", row.names = c(NA, -15L)) #tail(moredata) newggslopegraph(moredata,Date,Pct,Party, Title = "Notional data", SubTitle = NULL, Caption = NULL) #> #> Converting 'Date' to an ordered factor
There are a plethora of formatting options. See
?newggslopegraph for all of them. Here’s a few.
The most complex is
LineColor where you can do the following if you want to highlight the difference between the Liberal and NDP parties while making the other three less prominent…
Also from Tufte, this is data about a select group of countries Gross Domestic Product (GDP). I’ll use it to show you a tricky way to highlight certain countries without making a named vector with
LineColor = c(rep("gray",3), "red", rep("gray",3), "red", rep("gray",10)) the excess vector entries are silently dropped… The bottom line is that
LineColor is simply a character vector that you can fill any way you choose.
Finally, let me take a moment about crowding and labeling. I’ve made every effort to try and deconflict the labels on the left and right axis (in this example the
Country) and that should work automatically as you resize your plot dimensions. ** pro tip - if you use
RStudio you can press the
zoom icon and then use the rescaling of the window to see best choices **.
But the numbers (
GDP) are a different matter and there’s no easy way to ensure separation in a case like this data. There’s a decent total spread from 57.4 to 20.7 and some really close measurements like France, Belgium, and Germany on the right side. My suggestion is in a case like this one you create a new column in your dataframe with two significant places. So specifically it would be
newgdp$rGDP <- signif(newgdp$GDP, 2). In my testing, at least, I’ve found this helps without creating inaccuracy and not causing you to try and “stretch” vertically to disambiguate the numbers. This time I’ll also use
LineColor to highlight how Canada, Finland and Belgium fare from 1970 to 1979.
Then to demonstrate how flexible
LineColor really is I’ll use some
tidyverse tools to build a named list of countries and colors. The country’s line color will be determined by whether the difference between 1979 is positive, near neutral or negative.
custom_colors <- tidyr::pivot_wider(newgdp, id_cols = Country, names_from = Year, values_from = GDP) %>% mutate(difference = Year1979 - Year1970) %>% mutate(trend = case_when( difference >= 2 ~ "green", difference <= -1 ~ "red", TRUE ~ "gray" ) ) %>% select(Country, trend) %>% tibble::deframe() custom_colors #> Sweden Netherlands Norway Britain France Germany #> "green" "green" "green" "red" "green" "green" #> Belgium Canada Finland Italy US Greece #> "green" "gray" "green" "green" "green" "green" #> Switzerland Spain Japan #> "green" "green" "green" newggslopegraph(newgdp, Year, rGDP, Country, Title = "Gross GDP", SubTitle = NULL, Caption = NULL, LineThickness = .5, YTextSize = 4, LineColor = custom_colors )
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