'earliest start date'

My data consists of the details for classes/courses, e.g. date, location, number of enrolments etc. I want to be able to display each class/course in a stacked bar chart to visualise how many students are enrolled into each course by date and location. The problem I'm having is the same course may run on different dates and at different locations, and when I insert the course date into ToolTips, it will display the 'earliest start date', rather than the start date attached to the specific class. 

Each class has a unique code, and I have tried combining the course name and code into one column to no avail.