I have the code below. The intended behaviour is to set the default option in a set of radio buttons. I've used very similar code elsewhere and its worked but in this case the last 3 options are all showing as selected. I'm new to tkinter and can't for the life of me see where I've gone wrong. Any help much appreciated.

charttypevar=StringVar()

chart_type_1 = ttk.Radiobutton(chart_type_frame, text="3d with catagories",variable=charttypevar,value='3d with catagories')#
chart_type_2 = ttk.Radiobutton(chart_type_frame, text="3d no catagories",variable=charttypevar,value='3d no catagories')#
chart_type_3 = ttk.Radiobutton(chart_type_frame, text="3d relief plot", variable=charttypevar, value='3d relief plot')  #
chart_type_4=ttk.Radiobutton(chart_type_frame,text="Not yet defined",variable=charttypevar,value="Not yet defined")
charttypevar.set('Not yet defined')  # intended to be set as default

chart_type_1.grid(column=0,row=1,sticky="W")
chart_type_2.grid(column=0,row=2,sticky="W")
chart_type_3.grid(column=0,row=3,sticky="W")
chart_type_4.grid(column=0,row=4,sticky="W")