I understand that you are seeking guidance to add a vertical line in the Z direction within the YZ view of the mesh plot. Although you have attempted line annotation, it did not seem to be a feasible solution.
To address this issue, you can utilize the "line" function to plot a vertical line at the position y=0. In the following approach, I have plotted a line while keeping the X value fixed at its maximum, ensuring it appears in front of the mesh plot. Additionally, I extended a vertical line starting from [0, 0] to [min(Z(:)), max(Z(:))]. This method fulfills the requirements mentioned in the question. Here is the code snippet for your reference
% assuming random data
[X, Y, Z] = peaks(100);
subplot(1,2,1);
mesh(X, Y, Z);
subplot(1,2,2);
mesh(X, Y, Z)
view(90,0); % YZ view
hold on;
% add vertical line centered on Y-axis
line([max(X(:)) max(X(:))], [0 0], [min(Z(:)) max(Z(:))], ...
'Color', 'r', 'LineWidth', 2, 'LineStyle', '--');
hold off;
title('YZ View');
I hope this helps.
