Unit 9 B1: Choosing a chart for the question

Harbour Sports (a made-up shop) has twelve months of sales. The owner glances at a table of 12 rows and shrugs. Then she sees a line that rises and falls, and says: "Summer is our best time."

The numbers did not change. The picture made the answer easy to see.

Data visualisation means showing data as a picture. Its purpose is to make a pattern, a gap or a change easy to see, so a person can decide.

Many tools make charts. A spreadsheet draws one in a few clicks. A dashboard tool draws many from a store of data. A programming library draws charts from code. The choice of chart matters more than the tool.

A bar chart has one bar for each category. The longer the bar, the bigger the value. Use it to compare groups: sales for each product, members for each club.

A pictogram is a bar chart made of small pictures, each one standing for a set number. It is friendly but less exact.

-- One # for each unit sold, for each category.
SELECT products.category AS Category,
       SUM(sales.quantity) AS Units,
       REPLACE(HEX(ZEROBLOB(SUM(sales.quantity))), '00', '#') AS Bar
FROM sales
JOIN products ON sales.product_id = products.product_id
GROUP BY products.category
ORDER BY Units DESC, Category;

-- A charting tool draws the same bars in colour.