How to output command to command windows in the GUI CALLBACK?

 function plotdata_Callback(hObject, eventdata, handles)
% hObject    handle to plotdata (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
  evalin('base', 'plot(varString{2})');% code

The code can't plot a figure, I want to use workspace data to plot figure. Be equal to input plot(varString{2}); in the command windows.

[Stephen23]

There are several things about this code that suggest a less-than-optimal design:

• Using a callback to magically access data in the base workspace. Accessing data in another workspace not recommended, because it makes code slow, complex, and buggy (your question demonstrates this). A much better ("better" in the sense neater, more robust, much more efficient, less buggy) solution is to pass data to the GUI and process it within the GUI itself.
• The variable name varString seems to suggest that you want to magically access variable names. Magically accessing variable names is one way that some beginners force themselves into writing slow, complex, buggy code that is hard to debug:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

You would be much better off passing the data properly as input/output arguments, and processing it within the GUI.

See also your earlier question:

https://www.mathworks.com/matlabcentral/answers/416065-how-to-read-a-variable-whose-suffix-changes-randomly