You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: viz_skel.m
    path of file: ./repos/BPL/bottomup/skeleton
    
    the code of the file until where you have to start completion: classdef viz_skel
    % visualize the process of extracting the skeleton
    
    methods (Static)
        
        function plot_image(I)
            if UtilImage.check_black_is_true(I)
               
    
function plot_trace(I,T)
           % I : binary image
           % T : graph structure
            if UtilImage.check_black_is_true(I)
                I = ~I;
            end
            sz = size(I);
            hold on
            ns = length(T.S);
            
            image([1 sz(1)],[1 sz(2)],repmat(I,[1 1 3]));
            for i=1:ns
                stk = T.S{i};
                color = rand(3,1);
                plot_traj(stk,color);
            end