How can I measure value of points in matlab plot?

I have two data set. And I just need to determine 3dB Bandwidth. I want to locate value -4.01(Maximum is -1.01 and therefore this is the 3dB point on Y axis) on y axis and then corresponding x axis value. My code goes like this:

Insertion_loss = -1*[46.45,37.50,27.30,16.06,4.91,1.51,1.01,1.1,1.56,1.5,2.0,5.1,11.53,20.56,32.85,51.4];
Frequency= 1:0.2:4;
plot(Frequency, Insertion_loss)
xlabel('Frequency')
ylabel('Insertion Loss')
title('Frequency response of unknown filter')

[Walter Roberson]

Both of the -4.01 or just one of the two?

Answer 1

[Image Analyst]

Try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

Insertion_loss = -1*[46.45,37.50,27.30,16.06,4.91,1.51,1.01,1.1,1.56,1.5,2.0,5.1,11.53,20.56,32.85,51.4];
Frequency= 1:0.2:4;
plot(Frequency, Insertion_loss, 'bo-')
xlabel('Frequency', 'FontSize', fontSize)
ylabel('Insertion Loss', 'FontSize', fontSize)
title('Frequency response of unknown filter', 'FontSize', fontSize)
grid on;
xIndex = find(Insertion_loss > -4.01, 1, 'first');
f = Frequency(xIndex);
hold on;
plot(f, Insertion_loss(xIndex), 'b*');
% Draw lines to the curve.
xl = xlim();
yl = ylim();
line([f, f], [yl(1),  Insertion_loss(xIndex)], 'Color', 'r', 'LineWidth', 2);
line([xl(1), f], [Insertion_loss(xIndex),  Insertion_loss(xIndex)], 'Color', 'r', 'LineWidth', 2);

% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')