Help with plotting a specific point on a graph

I am pasting the function I am working on below. I added to it - just the part where the optimal h for backward difference is calculated. Can someone help me plot just the value hOptBackDiff on the graph as it exist?

I tried:

    loglog(h,err1,'ro',h,err2,'b*',hOptBackDiff,err3, 'k*', 'markers', 20);

Here are the calculations:

    % calculate optimal h for backward difference
    hOptBackDiff=2*sqrt((eps/abs(tdf2)));
    % not sure which possibility list to use
    % possiblehlist=[truedf2(x), truedf2(x-h)];
    possiblehlist=[truedf2(x), tdf2-hOptBackDiff];
    M2=max(possiblehlist);       
    hOptBackDiff=2*sqrt((eps/abs(M2))); % I want to plot this value in the graph below

The graphing portion of the function looks like this:

err1=abs(df-df1);
  err2=abs(df-df2);
  err3=abs(df-tdf2);

    % PLOT THE APPROXIMATION ERRORS AS A FUNCTION OF STEPSIZE
    figure
    loglog(h,err1,'ro',h,err2,'b*',hOptBackDiff,err3, 'k*', 'markers', 20);
%     loglog(hOptBackDiff,err3, 'k*', 'markers', 20); 

    % LINEAR REGRESSION OF LOG-LOG RELATIONSHIPS (ONLY IN REGION GOVERNED BY TRUNCATION ERROR)
    coef1=polyfit(log(h(end-4:end)),log(err1(end-4:end)),1);
    coef2=polyfit(log(h(end-4:end)),log(err2(end-4:end)),1);

    hold on % add the best-fit lines to the plot
    loglog(h(end-4:end),exp(coef1(2))*h(end-4:end).^coef1(1),'r-');
    loglog(h(end-4:end),exp(coef2(2))*h(end-4:end).^coef2(1),'b-');

    % LABEL THE PLOT
    set(gca,'fontsize',14) % be kind to the instructor's aging eyes!
    xlabel('h')
    ylabel('error')
    legend('back diff','central diff',...
        ['slope = ',num2str(coef1(1),4)],['slope = ',num2str(coef2(1),4)],...
        'Location','BestOutside');
    title(['at x = ',num2str(x)])

    %ALLOW USER TO VIEW EACH PLOT BEFORE MOVING ON TO THE NEXT
    if xi<length(allx)
        disp('Hit any key to continue...')
        pause 
    end
end
figure(gcf)

Thanks in advance!

[KSSV]

What problem you are facing now?

[mars]

It's posted above :/

[KSSV]

With out data we cannot run the code...rather specifying your error will give you a quick answer.