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Let's see why it's vital to visualize numbers by beginning with Table 1.1. There are four groups of numbers, each with 11 pairs. In a moment, we will create a chart from them, but before we do, take a look at the numbers. What can you see? Are there any discernible differences in the patterns or trends among them?

Table 1.1 Table with four groups of numbers: What do they tell you?

Let me guess: You don't really see anything clearly. It's too hard.

Before we put the numbers in a chart, we might consider their statistical properties. Were we to do that, we'd find that the statistical properties of each group of numbers are very similar. If the table doesn't show anything and statistics don't reveal much, what happens when we plot the numbers? Take a look at Figure 1.1.

Figure 1.1 Now can you see a difference in the four groups?

Now do you see the differences? Seeing the numbers in a chart shows you something that tables and some statistical measures cannot. We visualize data to harness the incredible power of our visual system to spot relationships and trends.

This brilliant example is the creation of Frank Anscombe, a British statistician. He created this set of numbers – called “Anscombe's Quartet” – in his paper “Graphs in Statistical Analysis” in 1973. In the paper, he fought against the notion that “numerical calculations are exact, but graphs are rough.”

Another reason to visualize numbers is to help our memory. Consider Table 1.2, which shows sales numbers for three categories, by quarter, over a four-year period. What trends can you see?

Table 1.2 What are the trends in sales?

Identifying trends is as hard as it was with Anscombe's Quartet. To read the table, we need to look up every value, one at a time. Unfortunately, our short-term memories aren't designed to store many pieces of information. By the time we've reached the fourth or fifth number, we will have forgotten the first one we looked at.

Let's try a trend line, as shown in Figure 1.2.

Figure 1.2 Now can you see the trends?

Now we have much better insight into the trends. Office supplies has been the lowest-selling product category in all but two quarters. Furniture trends have been dropping slowly over the time period, except for a bump in sales in 2015 Q4 and a rise in the last two quarters. Technology sales have mostly been the highest but were particularly volatile at the start of the time period.

The table and the line chart each visualized the same 48 data points, but only the line chart lets us see the trends. The line chart turned 48 data points into three chunks of data, each containing 16 data points. Visualizing the data hacks our short-term memory; it allows us to interpret large volumes of data instantly.