In small applications, maintaining expressions isn't a problem. Every time you want to modify something, you can open each chart and change the expressions. However, when you have many applications using the same or similar expressions, changing them one by one can become challenging.
A very simple and good way of maintaining expressions is to have them all in a centralized file. This can be an Excel file that you can then read. If anything changes, you must only change things in one place. The file also gives you a place to explain each of the expressions for further clarity.
Each expression will be stored in a variable, and the variable will then be used directly in the chart expressions.
The following is an example of an Excel file format containing the application expressions: