Every Tableau chart starts in the Data pane on the left. Fields above the gray line are dimensions: categories such as Region or Category that split the data into groups. Fields below the line are measures: numbers such as Sales that Tableau adds up.
Drag a dimension to Columns and a measure to Rows and Tableau draws one bar per group. It wraps the measure in an aggregation, so Sales becomes SUM(Sales). Blue pills are discrete and make headers. Green pills are continuous and make an axis.
The Marks card starts on Automatic. With a dimension on one shelf and a measure on the other, Automatic picks bars. Swap the two shelves and the bars turn sideways.
The practice data comes from Juniper Trail Outfitters, a made-up outdoor gear shop: about 1,700 order lines from 2022 to 2025. Click the Data Source tab at the bottom of the tool to look at the rows.