So, this data visualization thing is new to you, but you already know enough to avoid basic mistakes (pies, 3D…). While playing with the data, you make these two charts:

You already know that a bar chart helps you to compare data points, while a line chart is better at displaying trends, right? But you keep staring at them, not knowing which one to choose.  Suddenly, your little guardian angel whispers: “the bar chart is wrong”.

The bar chart? What’s wrong with the bar chart? And why is the line chart OK?

Then you realize that the vertical scale starts at 30, and apparently it should start at zero, so you change it in both charts:

That leaves a lot of white space under the line. The experts say you don’t have to start scales at zero if you are using line charts. So you revert the changes in the line chart:

Ah, yes! Now you can choose one of them. Your guardian angel agrees that both charts are correct. So, do it, pick one!

What’s the problem? Is there something bothering you?

Humm, I see. They don’t look that similar. You believe that people may draw different conclusions depending on the chart you choose.

OK, let’s discuss this a bit. Let’s talk about resolution.

What is chart resolution, anyway?

Higher resolution is usually a good thing. It means that you can see more clearly the difference between data points. To improve resolution in a chart, you zoom in, using the numeric scales:

How much you can zoom in? Well, the lower limit should be the first nice round number below the minimum value in your data set, and the upper limit should be (you’ve guess it) the first nice round number above the maximum value.

You have to make a little change to the rule when it comes to bar charts. In a bar chart, people compare heights, so if the bars are not proportional to the data they encode, you are misleading your audience. So, the charts above are both correct, but the one below is not:

That’s why the lower limit in a bar chart should always be the value that maintains the right proportions (usually zero). So, the take-away message is, improve resolution byt changing the scale, but in a bar chart you must keep proportions aligned with the data.

What about slopes?

Line charts are more subtle. Both charts below are correct:

The only difference between them is that the one on the left has a higher resolution, and in general is a better option considering Cleveland’s suggestions for banking to 45º (slopes should average around 45º). You can do it by changing the numeric scales and/or the chart aspect ratio. This is a suggestion (an excellent suggestion), but it also tells you that there is no strict rule to obey.

To be completely honest, I don’t care much about scales or aspect ratio in line charts, as long as they do not go overboard. What really matters is to have something to compare with. In the post Weltanschauung, Lies and Charts, I use these politically biased charts…

… and argue that only when you have more than one series you can learn anything from a line chart, like this:

So it doesn’t matter much what you do with scales or  aspect ratio, as long as you have two or more series and your goal is to compare them.

You can learn more about scales in Naomi Robbin’s Creating More Effective Graphs and she often writes about it, like in here and here.

Perhaps you could use this as a rule of thumb: use a bar chart when you have a single series and a line chart when you have two or more series. It will not always work, but it’s a good starting point, don’t you think?

By the way: “méfiez-vous des morceaux choisis” is a lovely French expression that roughly translates to “beware of selected pieces”.