Posts filed under Graphics (317)

May 22, 2015

Budget viz

Aaron Schiff has collected visualisations of the overall NZ 2015 budget

A useful one that no-one’s done yet would be something showing how the $25 benefit increase works out with other benefits being considered as income — either in terms of the distribution of net benefit increases or in terms of effective marginal tax rate.

May 20, 2015

Weather uncertainty

From the MetService warnings page


The ‘confidence’ levels are given numerically on the webpage as 1 in 5 for ‘Low’, 2 in 5 for ‘Moderate’ and 3 in 5 for ‘High’. I don’t know how well calibrated these are, but it’s a sensible way of indicating uncertainty.  I think the hand-drawn look of the map also helps emphasise the imprecision of forecasts.

(via Cate Macinnis-Ng on Twitter)

May 5, 2015

Civil unions down: not just same-sex

The StatsNZ press release on marriages, civil unions, and divorces to December 2014 points out the dramatic fall in same-sex civil unions with 2014 being the first full year of marriage equality. Interestingly, if you look at the detailed data, opposite-sex civil unions have also fallen by about 50%, from a low but previously stable level.


April 24, 2015

Graph of the week

Via @ian_sample on Twitter, a UK election ad


The basic approach has been traditional with the Liberal Party since before they merged with the Social Democratic Party; the accuracy has been, let’s say, variable. In this example, the 19-point difference between Labour and Liberal Democrats is shown as larger smaller than the 5-point difference between Liberal Democrats and Conservatives.

Here’s what those numbers really look like:



April 14, 2015

Cumulative totals go up

From ThinkProgress  (graph from Wikipedia) “U.S. plug-in electric vehicle cumulative sales have soared in the past few years, thanks in part to rapidly falling battery prices” and “A major reason for the rapid jump in EV sales is the rapid drop in the cost of their key component -– batteries.”


From a cumulative graph it’s hard to tell whether the cumulative sales have soared due to rapidly falling battery prices or just due to the fact that cumulative sales have to increase, but the past few years look pretty much like straight lines to me.

Here’s the noncumulative monthly sales, with the same colour-coding: there hasn’t been a big increase in the rate of sales during 2013 or 2014, so it’s not clear there’s much for falling battery prices to explain. Beyond the graph, for the first three months of 2015 there have been slightly few sales than in the first three months of 2014.


Cumulative sales of a new technology with sizeable network effects are important: it matters how many plug-in vehicles are out there. A cumulative graph is still a bad way to see patterns.


April 9, 2015

Graph of the week


Number of learner license tests taken in New Zealand, according to One News.

We’ll follow up to see if the future prediction part of the graph turns out to be correct.

April 7, 2015

Evils of Axis

First, from Mother Jones magazine, via Twitter


The impact of the carbon tax looks impressive, but this is a bar chart — it starts at zero and they’ve only shown the top fifth of it.

They do link to the data, the quarterly Greenhouse Gas Inventory update.  In that report, Figure 8 is


The dotted line is the same data as the bar chart, except that the dotted line has data for every quarter and the bar chart has data only for the July-September quarter each year. And  the line chart has a wider range on the vertical axis — it doesn’t go down to zero, but it isn’t a bar chart, so it doesn’t have to. The other point about the line chart is that there’s a solid line there as well. The solid line is adjusted for seasonal variation and weather. If you wanted to know about real changes in how Australians are using energy, that’s the line you’d use.


Second, a beautiful map of CO2 emissions from fossil fuel combustion, from the Washington Post via Flowing Data


The ‘vertical’ scale here is a colour scale; what’s misleading is that it’s a logarithmic scale. The map makes it look as if a large fraction of CO2 emission comes from transporting stuff through empty areas, but the pale beige indicates emissions thousands of times lower than in the urban/suburban areas. Red ink isn’t anywhere close to being proportional to CO2.

March 30, 2015

Aspect ratios and not starting at zero

The vertical axis on a bar chart must start at zero. The very rare exceptions are ones that prove the rule: where ‘zero’ isn’t zero. Otherwise, the axis starts at zero or it isn’t a bar chart. The whole point of bar charts is that the length of the bar is proportional to the data value.

Line charts and scatterplots are different.  They don’t need to be tied down to zero, and the axis scales can be chosen to make the information as clear as possible. With great power comes great responsibility, as we can see from the following pair of line graphs of oil drilling in the US.



It’s pretty obvious that these come from people with different communications agendas. Or, it would be, except they are from the same story at Bloomberg.

Neither graph has an ideal aspect ratio. The flat one is too flat: you can’t see the wobbles over time in number of rigs. The tall one is too tall: the number of rigs has halved, but it looks as though it has crashed much more than that.

Bill Cleveland has a useful default rule for scaling line graphs: the median slope of the line segments should be about 45 degrees. The orange line on the tall graph isn’t far off that, but the blue line is steeper.  The 45-degree rule would give a graph like this:


In fact, there is plenty of room to start the blue axis at zero, but that’s not always the right choice.

Here, in a sadly-appropriate pairing, is the Keeling Curve, the graph of atmospheric CO2 concentrations at Mauna Loa observatory, in a visualisation paper from Berkeley.


There’s no sense at all in having the vertical axis start at zero. Zero is just not a relevant value of atmospheric CO2. What’s more interesting, though, is how the two scalings show different information. The upper graph is scaled so the year-to-year changes have slope centred at 45 degrees. This makes it easier to see that the CO2 increase is accelerating. The lower graph is scaled so the month to month changes have slope centred at 45 degrees, making it easier to see the shape of the seasonal pattern.

Different vertical scaling can be used just to mislead the reader, but it can also be used to make data more readable and to communicate more effectively.

March 23, 2015

Cricket visualisations


Population genetic history mapped

Most stories about population genetic ancestry tend to be based on pure male-line or pure female-line ancestry, which can be unrepresentative.  That’s especially true when you’re looking at invasions — invaders probably leave more Y-chromosomes behind than the rest of the genome.  There’s a new UK study that used data on the whole genome from a few thousand British people, chosen because all four of their grandparents lived close together.  The idea is that this will measure population structure at the start of the twentieth century, before people started moving around so much.

Here’s the map of ancestry clusters. As the story in the Guardian explains, one thing it shows that the Romans and Normans weren’t big contributors to population ancestry, despite their impact on culture.