I made a column representation in QGIS, which shows the effects on different points around point 24.
Is it possible to label the columns on top with the unit as you are used to from excel?
I've tested some options (QGIS 3.10). It's not really straight forward, and you will need to write quite a few expressions, with some math involved.
Diagram size need to be in "Map units" (also in rendering). Bar length is then used as a base for a lot of expression values later.
The expressions need to consider the diagram dynamic size. This is fractions of the bar length and bar width.
Use data defined placements for each label, also based on the diagram size and defined placement using the geometry. Think of it as moving each label from the geometry to the correct location using the placement and size of the diagram, if that helps as an explanation.
This may actually be easier, but it's your choice. However, note that screen x,y is measured from top left, and coordinate x,y from bottom left. This means your "y" offset will be inverted compared to using "Map units".