Since you are able to obtain the variable in workspace, there must be some way to pass it to the MATLAB App designer (say via a function call, or running the process that gets you the variable inside app designer, etc.). Once you get the variable stored in app designer:
1) Drag and drop a button component and Axes component in app view. Click on the button. Click 'callbacks' on the right pane. Add a buttonPushed callback which takes you to the boilerplate code.
2) Add:
plot(UIAxes, variableYouWant)
