Often, you'll need to use more than one axis to compare different measures, understand correlation, or analyze the same measure at different levels of detail. In these cases, you'll use visualizations with more than one axis.
A scatterplot is an essential visualization type to understand the relationship between two measures. Consider a scatterplot when you find yourself asking questions like these:
Does how much I spend on marketing really make a difference to sales?
How much does power consumption go up with each degree of heating/cooling?
Is there any correlation between rental price and the length of contract?
Each of these questions seeks to understand the correlation (if any) between two measures. Scatterplots are great to see these relationships and also to locate outliers.
Consider the following scatterplot, which looks at the relationship between the measures of the sum of Sales (on the x axis) and the sum of Profit ...