How Do I Create a Performance-Potential Matrix for Career Planning in Middle Management?

I've given a project to do career planning for middle management level employees. Can somebody tell me how to draw a performance-potential matrix? On the x-axis, we can take performance as one parameter. For data, we can use the ratings of performance appraisal. But what about potential? Do we need to design a test? Please help me out. Thanks.

I am also working on the Performance-Potential Matrix, wherein we need to create a pool of potential employees whom we can promote to the next level for future use. Can anyone suggest the criteria for identifying potential employees? Or if anyone can forward a format for the Potential Matrix, it would be of great help.