When you’re going to compare two metrics of comparable sizes in a XY plane, it’s sometime useful to add a 45 degrees reference line (in geometrical terms, a bisecting line). This is extremely useful, among other cases, when you need to compare an above-the-line/below-the-line performance. A typical example, discussed here, is when you need to compare if the channels of incoming visitors to a website are more capable of attracting new rather than returning visitors.
Technically speaking, Tableau doesn’t allow to draw diagonal reference lines, but this could be done with a very simple trick I’m going to explain here. I tested on Tableau Professional 9.0, but it should work with other versions as well.