Now i have a question that I write a draw() function in the gui tab to do some data visualization.

If you look at the tpp of the gui tab code you will see these comments

/* =========================================================
 * ====                   WARNING                        ===
 * =========================================================
 * The code in this tab has been generated from the GUI form
 * designer and care should be taken when editing this file.
 * Only add/edit code inside the event handlers i.e. only
 * use lines between the matching comment tags. e.g.

 void myBtnEvents(GButton button) { //_CODE_:button1:12356:
     // It is safe to enter your event code here 
 } //_CODE_:button1:12356:
 
 * Do not rename this tab!
 * =========================================================
 */

it quite clear that you can't add functions in the gui tab - put your draw function in the main sketch tab.