Hi
Welcome to the board
1 - Insert a chart title:
Ribbon->Chart tools->Layout->Chart title
2 - with the axis title box selected (just selected with a continuous border line, not in edit mode with dashed border line) write the equal sign in the formula bar, click on a cell and confirm with Enter. You'll get in the formula bar something like:
=Sheet1!$E$1
Now the axis title will display the value in Sheet1!$E$1