The meter box has so far been only a set of VUs, but my intention is to interpret a range of MIDI message types into different displays. A few days ago I got the pitch wheel message display working, though it has no PNG background image yet.
The pitch wheel message carries 14-bit value, that is interpreted as positive or negative. Typically the full scale positive and negative values will result in a pitch change up or down one octave.
This is quite straightforward to convert into a display, using the same needle as with the VUs. A few parameters in the needle line drawing function needed to be changed to get the needle to move in a way appropriate to the task.
There was one surprise. I had just bought the keyboard controller through craigslist, and was not familiar with its operation. At first I was thrown by the MIDI activity, so I used my protocol analyser to have a look. It turned out this device uses MIDI's running status. With running status, the device sends the pitch wheel command once, then any number of the 14-bit values after that, as long as no other MIDI messages needing to share the cable. So running status was coded into the message interpreter.