It is all about giving that little bit more insights which can be done in many ways. For example, you want to show the sales by product category, which you’ve put in a bar chart. Looking at these bars, you might be interested in the number of manufactures involved in these sales amounts for product category. You can create a stacked barchart representing the different manufacturers in a legend. Or you can use another chart in your report to represent the top 5 products, which will interact with the sales over time chart. But both options will use additional space on your report canvas and look a bit messy, which can distract the users of where it is all about.
Marc is wrapping up the series and it’s worth the read.