Splitting/Combining Data for Helpdesk to drill down

Hello everyone!

I need to desktop our helpdesk logging system (Track-IT) and When it comes to assigning tickets there are three columns, which we will call Department, Task and Topic.  So the Department column has the teams, like the helpdesk, Deployment and application specific support teams.  Under that we have the Tasks, which will be like Install something or Reset a password. Then under that is the Topic, so if the Task is Reset Password, the Topic will be AD Password, Email password and so on.

So I am wondering how I can express that data with a visualisation? So if I start of with a graphic that shows the department and how many calls they get, I think I then want to tap on one of them, for it to switch to showing that department's Tasks, and then drill down to the topic.  I appreciate the data will need a lot of work, but I can't even start that, as I still cannot envisage how to present the data. Most of the examples relate to simple helpdesks that may just concern one product, and none of them help identify trends, most are just visualising counts witht he odd graph to plot busy periods.

So, has anyone done anything similar or have a good suggestion ofor an intuitive visualisation??