In an application I'm working on I wanted to draw my own shapes based on the tab as well. The shapes are not controllers so you can't move them to a tab like you do with those. The solution I chose is to have a variable keep track of the active tab. You can give each tab an id and when you set activateEvent for the tab, you can set the id when you click each tab. When you know which tab is active, you can use that information to draw the non-gui stuff you want to draw in each tab.

Adapted Code

1. import controlP5.*;
2. ControlP5 p5;
3. int activeTab;
4.  
5. void setup() {
6.   size(400, 300);
7.   smooth();
8.   p5 = new ControlP5(this);
9.   p5.getTab("default").setLabel("rectangle").activateEvent(true).setId(0);
10.   p5.addTab("ellipse").activateEvent(true).setId(1);
11. }
12.  
13. void draw() {
14.   background(120);
15.   if (activeTab == 0) {
16.     rect(80, 50, 55, 55);
17.   } else if (activeTab == 1) {
18.     ellipse(150, 150, 100, 100);
19.   }
20. }
21.  
22. void controlEvent(ControlEvent theControlEvent) {
23.   if (theControlEvent.isTab()) {
24.     activeTab = theControlEvent.getTab().getId();
25.   }
26. }