It seems that you access the value of the button, which is not what you intend to do. What you probably want to do is to share data among callbacks: https://de.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html
In your two functions you could add something like
data = guidata(hObject);
to retrive the data and
guidata(hObject, data)
to store data. The guidata can only exist of a single variable, but this variable could be a struct which stores different data as fields.
You can add the following to the button callback to store the sample data
data = guidata(hObject);
data.Unexcited_Sample_file = Unexcited_Sample_file;
guidata(hObject, data)
And the following to the plotting function
data = guidata(hObject);
Unexcited_Sample_file = data.Unexcited_Sample_file;
Please also consider to initialize the struct variable (e.g. in gui opening fcn)
