It’s not something I would have thought of myself, but a German magazine recently tried it – and ever since, I’ve been wondering whether this is a clever idea or actually a very bad one. ChatGPT kindly translated the chart for me; the translation isn’t entirely accurate, but that’s not the point. What matters here is the visualization approach. In short, the data is about partnerships between the public sector and private investors.
A quick reminder: the purpose of data visualization is to make a subject easier to understand. Ideally, it also carries an intention – something that sharpens our perspective, changes our opinion, or even nudges us toward action. So, what does this visualization achieve?
At first glance, we see a stacked column chart mapped to the left Y-axis. In addition, a gray line shows the cumulative number of projects, plotted against the right Y-axis. I’m not a fan of dual axes, as they rarely make interpretation easier unless there is a clear pattern. That could be the case, for example, if one axis showed annual marketing investments and the other the resulting revenues – in other words, if the relationship were obvious.
But even if there were such a relationship here, the chart wouldn’t reveal it. That’s because the designers chose a particularly tricky representation for the line: cumulative values. Instead of showing the number of projects per year, the line shows the total number of projects accumulated since the start of the period. If you want to demonstrate a correlation, you need to use the same type of measure – either both cumulative or both annual flows.
Perhaps the intention was different: maybe the cumulative line was meant to show the number of projects still active at each point in time? Neither the legend nor the article makes this clear. That interpretation would at least make sense, since public-private partnerships cover not just the construction of, say, a highway, but also its ongoing operation.
On closer inspection, though, the questions multiply. Between 2003 and 2006, investment volumes rose sharply, so the cumulative number of projects should also have risen more quickly – after all, earlier projects would still be included in the total. Does that mean projects simply got more expensive? Or that larger, more complex projects were undertaken? Or, if the line represents only active projects, does it imply that some projects were completed and dropped out of the total? That seems unlikely: highways, for instance, don’t pay for themselves within just a few years.
Around 2012, there’s a dip (in the translated chart a few columns are even missing), and afterward the number of projects rises more slowly – even after 2020, when record sums were invested. The expected effect – more money, more projects – just isn’t visible. Why not? The chart doesn’t say. In fact, I’m left more confused than before, and that’s not exactly a hallmark of good visualization. Even if the magazine simply reproduced the chart from the original source, I would have hoped for a bit more thought here.
Which brings us to the second requirement: intention. What exactly is this chart trying to say? That public-private partnerships are a great idea? For whom? (The article itself mentions that investor-driven projects often cost more, since they must deliver higher returns, and that there have been capital losses as well.) Am I supposed to support these partnerships? Oppose them? Consider reforms? The chart gives me no clue.
My guess is that this visualization belongs in the category of “let’s break up the text with a chart” rather than “let’s help the reader understand the data.” I can’t see another explanation. Technically weak, muddled by the use of cumulative values, and devoid of a clear message. The end result is a chart that confuses more than it clarifies – which is the very opposite of what good data visualization should do.