How to know interpolated points in a MATLAB plot?

Hi everyone,

I have the plot of data received from a VNA. To read the bandwidth I need to know the coordinates of the points that have the y coordinate -10. This data is not present in the VNA data sheet, but MATLAB interpolates it with the plot function. How can I find the points with coordinate y = -10 from the code?

Thanks a lot everyone!

Answer 1

[Bruno Luong]

% Dummy example of some curve that cross the horizontal line y=10
x = cumsum([0 rand(1,40)]);
y = 12*sin(x);
plot(x,y,'.-');

hl = findobj(gca, '-depth', 1, 'type', 'line');

x=get(hl,'xdata');
y=get(hl,'ydata');

y10 = -10;
i = find((y(1:end-1)-y10).*(y(2:end)-y10) < 0);
ipick = i(1); % select where it cross and where x is (in your graph it has at lzast 2 crossing points)
x10 = interp1(y(ipick:ipick+1),x(ipick:ipick+1),y10)

x10 = 4.1678

hold on
xline(x10)
yline(y10)
plot(x10,y10,'ro','Markersize',10)

[Alessandro Picca]

Thanks a lot! It's what I want!