Course data up on a tree(map)

The budget of Finland has seldom given a more lavish yet concise visualization than Talousarvio 2012 by Juuso Pääkkönen. I wanted to try something at least remotely similar with the Aalto open course data.

Treemap of Aalto courses by School and department

My map does not offer any interactive elements and doesn’t necessary tell much at the outset, so here a few words of explanation.

The big rectangles are Schools, further broken down into smaller rectangles, departments. The bigger the rectangle, the more courses there are. Why is ECON a one big block?
Course codes of BIZ aka former ECON are not mapped to the department level like is the case with all other Schools.

Courses do not reveal much numeral data like a budget but something still: ECTS credits tell about the workload of the course. In the map, credits are averaged by the department. In case ECTS is given as a range, say 1-10, I have used the upper value only.

Here, a course object can be anything from a one week seminar to a doctoral thesis.

The R code is available as a GitHub Gist.

EDIT 1: 92 courses are given ECTS 0 credits. 61% of these belong to ECON. In this exercise, I haven’t excluded these.
EDIT 2: In the map, ERI stands for Other separate courses given by other units than the Schools per se
EDIT 3: the map is incorrect, see the newer post