Help with column stacking

Hi and thanks in advance for helping a very green newbie.

Simple charting questions. I want to represent the following data in a stacked chart that will be tracked monthly:

total number of outstanding tasks
number of tasks overdue
overdue tasks as a percentage of total

When entering the data in excel, the total number of tasks and the number of overdue tasks keep getting added together. What i need is the overdue tasks highlighted as a portion of the total task stack and then to indicate that this 'substack" is x% of the total.

Hope this makes sense and thanks in advance for the assistance.