Hi, there are several way to do what you want.
BTW I cant see the linked image, you can post it directly here in the forum.
Out of all the ways you have to draw the one you are using is the slowest one, this is because each for each call of ofVertex there's an openGL call, thus sending data to the gpu. It's always faster to use something like an ofMesh or an ofVboMesh and then modify via openGL shader.
There's an example that's very much what you are trying to do in the tutorials folder in your openFrameworks install.
Take a look at it.
best!