There are so many elements in a chart which can be changed in so many different ways. The question is, which menu (tab) to choose for what purpose? You may not have thought about it before, now is a good time to understand this.
Anything which talks about the WHOLE chart is there in Design tab.
Anything which changes one of the elements of the chart is in the Layout tab. Axis, labels, series, etc. In the new version, all options under Layout were added to the Design tab.
Finally, everything in a chart is just a shape. So if you want to customize anything like a shape – background color, border, shadow, font, etc. then use the Format tab.