There are two modes when rendering a visualization, render row by row, or render by grouping.
Consider the following SharePoint list:
In this example, we simply want to visualize every row of data on a timeline, like this:
In this example, suppose we are interested in the average market share per browser, in this case, we want to group the data by browser name and visualize the average value. Like this:
This mode is called Render by Grouping, The result: