Why we draw data at all

By the end of this lesson you can explain what a chart shows faster than a table, and what it hides.

Why it matters

Busy people often glance at a chart for a few seconds instead of reading a table. If you can turn numbers into a clear picture, people will actually use your work.

The idea in plain words

Here are Amara's sales for each day of the week, as a small table. Which day was the busiest?

To answer, you have to read all six numbers and compare them in your head.

You can do it, but it takes a moment. Your eyes jump from number to number.

Now imagine the same six numbers drawn as bars, where each bar's height is the number. You will see it in a moment. The tallest bar would jump out straight away.

That is the power of a chart: your eyes are very good at comparing heights and lengths, much faster than they compare written numbers.

Chart: a picture of data, where sizes, heights or positions show the numbers. Also called a graph.

See it done

Data: Amara's sales per day, from market-stall-week.csv (synthetic). Each number is a COUNTIF, as in lesson 3.5: =COUNTIF(C:C,"Mon") and so on.

Question: what does the chart show quickly, and what does it hide?

The same six numbers as bars. Friday jumps out straight away: it is the tallest bar.

1. Busiest day. From the chart: Friday, with 12 sales. From the table you get the same answer, but more slowly.
2. Quietest day. From the chart: Wednesday, the shortest bar, 8 sales.
3. The pattern. The chart shows at a glance that sales went down from Monday to Wednesday, then up to Friday.
4. What it hides. The chart does not show which fruits were sold, when in the day, or how people paid. Each bar squashes many rows into one number.
5. Exact values. This chart prints the number on each bar. Many charts do not. Then you can only read values roughly from the scale of numbers at the side (called the axis; more on it next lesson).

So: a chart is great for comparing and spotting patterns. A table is better for exact values and detail. Good analysts use both.

Your turn, with help

Look at the chart again. Which two days had the same number of sales?

Hint: look for two bars that reach exactly the same height, then check the numbers printed on top.

Your turn, alone

1. Using only the chart, was Thursday busier than Tuesday?
2. How many more sales did Friday have than Wednesday?
3. Name one question the chart cannot answer, but Amara's full table can.
4. In Amara's tab, check the Friday bar with a formula: type =COUNTIF(C:C,"Fri") in an empty cell. Does it match the bar?
5. A manager has 10 seconds. Would you show her the table or the chart to show which day was busiest? Why?

Watch out

• Charts without numbers. If the bars have no values printed, read them from the axis and say "about". Do not pretend to more accuracy than the chart gives.
• Forgetting what is hidden. A bar is a total. It can hide very different things inside it.
• Decoration. 3D bars, shadows and pictures make charts harder to read, not easier. Simple is clearer.

Say it back

1. What is a chart?
2. What is a chart better at than a table?
3. What is a table better at than a chart?

What is next

Every chart has the same few parts: a title, an axis, labels, bars and a source. Miss one, and you can misread the whole thing. Next lesson, on real data about 17 countries.