fltk error

[coru]

I have the trouble of swithing from "qt" to "fltk".

If I tried use fltk and type the following, 
>graphics_toolkit ('fltk'),
I got return error message like below.
>error: graphics_toolkit: fltk toolkit is not available
>error: called from
>    graphics_toolkit at line 81 column 5

for other info:
>available_graphics_toolkits 
>ans =
>{
>  [1,1] = gnuplot
>  [1,2] = qt
>}

I already installed fltk.
>brew install fltk
>Warning: fltk 1.3.5 is already installed and up-to-date
>To reinstall 1.3.5, run `brew reinstall fltk`

I also tried to specify the fltk variant like following
>sudo port install octave-devel +fltk+carbon+atlas+docs
but I got same error.

I am using macOS (catalina 10.15) and 
GNU octave version is 5.1.0.

Could anyone help this issue, please?





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