The GUI component of the chord progression section is slightly more evolved than the previous two sections. Here's how a typical chord progression GUI looks like:
Notice that this section has the combobox as opposed to two for the earlier sections. Depending on what progression is chosen in the middle combobox, we need to draw a number of buttons, each button representing one chord in the complete chord progression.
In the preceding screenshot, note that the progression combobox has a value of I-V-vi-IV. This is a total of four roman numbers separated by a dash. This means that this chord progression is ...
