interp2() 'monotonically increasing' error

I have many lines that represent a "z" value and want to interpolate at a specific value between them, at a specific x-axis location, to get the y-value. I'm trying interp2() but it throws a 'monotonically increasing' error.

The dataset below is a subset. I broke it out into xyz-1 and xyz-2 just for easy plotting in this question (i.e., making a repeatable example). How can I fix my interp2() or the inputs?

x1 = [0.02, 0.048, 0.108, 0.196, 0.279, 0.401];
y1 = [0.583, 0.43, 0.32, 0.279, 0.262, 0.259];
z1 = [50, 50, 50, 50, 50, 50];

x2 = [0.02, 0.048, 0.108, 0.196, 0.279, 0.401];
y2 = [0.747, 0.591, 0.435, 0.357, 0.326, 0.305];
z2 = [35, 35, 35, 35, 35, 35];

x_all = [x1, x2];
y_all = [y1, y2];
z_all = [z1, z2];

plot(x1, y1, 'blue', 'DisplayName', 'z1')
hold on
plot(x2, y2, 'magenta', 'DisplayName', 'z2')
xlabel('x') 
ylabel('y') 
legend


want_x = 0.2;
want_z = 40;
need_y = interp2(x_all, y_all, z_all, want_x, want_z, 'linear')

Error:

Error using griddedInterpolant
The grid vectors must be strictly monotonically increasing.

Error in interp2>makegriddedinterp (line 228)
    F = griddedInterpolant(varargin{:});

Error in interp2 (line 128)
        F = makegriddedinterp({X, Y}, V, method,extrap);

Answer 1

[Walter Roberson]

interp2() is only for two-dimensional interpretation over a grid, not for interpolation of vectors. You need something like

F = scatteredInterpolant(x_all, z_all, y_all, 'linear');   %NOT y_all, z_all

need_y = F(want_x, want_z);

[newbie9]

thank you, worked like a charm (sans a small typo, scatteredInterpolatn). and thank you for including the commented remined on the order