The tricky bit is getting a space between the bars to add the text.
There may be better suggestions out there but the only way I know how to do this is if each bar is an expression. You can then put in a dummy expression of '= 0' to create the space.
Then you can overlay the bar chart with a transparent text box containing the calculation that you wish to appear.
This is not a great solution and there may be other methods out there but it gets the job done.