Choosing the right chart¶

Exercise choosing-the-right-chart-annex.pdf · PDF

You have built six kinds of chart by now: bar, line, histogram, scatter, pie and heatmap. Each time, somebody told you which one to make. Now a new question lands on your desk: "Do better-rated games sell more?" Which of the six do you open?

Most people answer with the chart they like best. That is choosing a tool by its colour. In this lesson you stop choosing by taste: the question decides, and you learn to read what it is asking for.

The block decides the tool¶

In the crafting game of this module, nobody mines stone with a shovel. The block in front of you decides: pickaxe for stone, axe for wood, shovel for dirt. Mining, chopping and digging are different jobs, and each job has its tool.

Charts work the same way. We keep this analogy for the whole page:

So every question gets read twice before you touch Superset:

• What kind of question is it? There are five jobs.
• Which columns does it touch, and of what type?

Get those two answers and the chart is no longer a matter of opinion.

Five jobs a question can ask for¶

The type of question is what the reader wants to do with the numbers: rank them, follow them in time, see how they spread, see if two of them move together, or see how a whole splits into parts.

Here are the five jobs, plus a sixth panel you will meet in a moment, all drawn from the same table of 26,685 Steam games:

One table, six completely different pictures. None of them is "the chart of the games". Each one is the answer to a different question.

Read the material: three types of column¶

A job is only half of the decision. A pickaxe is for mining, but you still need to know whether the block is stone or obsidian.

The data type of a column says what kind of values it holds. For choosing a chart, three types are enough: category, number and date.

Two columns can both be categories and still behave very differently. owners_tier has 3 values. publisher has 13,951. Keep that number in mind: it is the trap of this lesson.

The decision table¶

Put the two readings together and you get the table below. Keep it next to you: it is the cheat sheet for the rest of the unit.

The same table as a path you can follow with your finger:

flowchart TB
    Q["<b>The question</b><br>(the block in front of you)"] --> ONE{"Is the answer one number,<br>or a list of exact values?"}
    ONE -- "yes" --> BIG["<b>Big Number or Table</b><br>no chart needed"]
    ONE -- "no" --> JOB{"What is the job?"}
    JOB -- "who has more?" --> CMP{"How many<br>category columns?"}
    CMP -- "one" --> BAR["<b>Bar Chart</b>"]
    CMP -- "two" --> HEAT["<b>Heatmap</b>"]
    JOB -- "how did it change<br>over time?" --> LINE["<b>Line Chart</b>"]
    JOB -- "how are the<br>values spread?" --> HIST["<b>Histogram</b>"]
    JOB -- "do two numbers<br>move together?" --> SCAT["<b>Scatter Plot</b>"]
    JOB -- "what share is<br>each part?" --> PARTS{"How many parts?"}
    PARTS -- "5 or fewer" --> PIE["<b>Pie Chart</b>"]
    PARTS -- "6 or more" --> SBAR["<b>Bar Chart</b>, sorted"]

Trap 1: a whole cut into too many parts¶

"How is the catalogue split among publishers?" The job is composition, so your hand goes to the pie. Stop and count the parts first:

SELECT COUNT(DISTINCT publisher) FROM games_clean;

The answer is 13951. Try to cut a circle into 13,951 slices. The job is right; the material does not fit the tool.

Compare two compositions. The first has three parts and is a perfectly good pie:

pie showData
    title Steam games by owners tier
    "Niche" : 17921
    "Mid" : 8141
    "Hit" : 623

The second is how the 949 Pokémon of the class table split by primary type. Eighteen parts. As a pie it would be eighteen thin slices and a legend you read with a ruler. As a sorted bar chart you see the answer at once: water and normal lead, flying is almost empty.

%%{init: {"themeVariables": {"xyChart": {"plotColorPalette": "#4051b5"}}}}%%
xychart-beta horizontal
    title "Pokémon per primary type (18 parts: sorted bars, not a pie)"
    x-axis [water, normal, grass, bug, rock, psychic, electric, fire, ghost, dragon, dark, ground, poison, fighting, steel, ice, fairy, flying]
    y-axis "Number of Pokémon" 0 --> 130
    bar [126, 111, 84, 79, 65, 64, 61, 59, 40, 39, 37, 36, 35, 31, 30, 29, 19, 4]

The rule: with five parts or fewer, a pie. With six or more, compare the biggest parts with a sorted bar chart. For the publishers, that is exactly the top-10 bar chart you built in Bar and line charts: same columns, honest tool.

Trap 2: a question that needs no chart¶

"How many games are there in the table?"

SELECT COUNT(*) FROM games_clean;

26685. One number. There is nothing to rank, nothing to follow in time, nothing to spread out. Not every block needs a diamond pickaxe.

A chart is for seeing a pattern. If there is no pattern to see, a chart only slows the reader down.

Practice: eight questions, eight decisions¶

Before opening Superset, try these on paper. For each one write three things: the job, the columns and their type, the chart. Two of the eight are traps.

Try it: build two of them¶

Questions 1 and 2 you already own from Bar and line charts. Questions 6 and 4 are new blocks: you know the tools, and now you know why to pick them. You need the games_clean dataset in Superset (the files are on the Resources page).

Question 6: price band by owners tier. Two categories and one number: a heatmap.

Question 4: Metascore against owners. Two numbers: a scatter plot.

Key takeaways¶

• The question picks the chart, not your taste. Read it twice: the type of question (the job) and the data type of the columns (the material).
• Five jobs: comparison (bar), evolution (line), distribution (histogram), relationship (scatter), composition (pie).
• Three column types are enough: category, number, date. A year is a point in time, not a measured number.
• Two categories and one number is a heatmap: a comparison made two ways at once.
• A composition with six parts or more is not a pie: use a sorted bar chart.
• If the answer is one number or a list of exact values, show a Big Number or a Table. Not every question needs a chart.

Quick check¶

You can now pick the right tool for any block. But the right chart can still be unreadable: no titles, an axis in 2k and 4k, a legend that says COUNT(*). Next: Configuring charts in Superset, where the chart learns to explain itself.