Measuring distance in 2D plot

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Hello everyone,
I am attempting to measure the horizontal length between two almost parallel streamlines. I would like to simply draw a line on the plot and have it display the length in the X-direction (not the length of the line itself). Is this an easy thing to do? I could not find it in the plot tools. I have attached the code and the mat file used in this.
Edit: In the photo below, I show what I have been attempting to do.
% PIV streamlines code
clc;clear;close all
file = dir('*.txt');
filename =;
data = readmatrix(filename);
% Min and max of the data x and y
kx = 0; % mm - Shifting factor
ky = 0;
spanX = [min(data(:,1))+kx max(data(:,1 ))+kx];
spanY = [min(data(:,2))+ky max(data(:,2 ))+ky];
% Generate equally spaced x and y arrays
Xq = linspace(spanX(1),spanX(2),640);
Yq = linspace(spanY(1),spanY(2),540);
% Create the combination of xq and yq
[Xq, Yq] = meshgrid(Xq,Yq );
% Interp the data if the x and y data in the file is not in a grid
Uq = griddata(data(:,1),data(:,2),data(:,3),Xq,Yq);
Vq = griddata(data(:,1),data(:,2),data(:,4),Xq,Yq);
xtip = -4.5;%-1.5; % Bottom left vertice of left probe wall
ytip = -7.703; % Bottom left vertice of left probe wall
height = 12.7; % mm
width = 1; % mm
gap = 3; % mm - Distance between probe walls (left end of the walls)
%% Plot vectors, probe walls, and streamlines
% quiver(Xq,Yq,Uq,Vq, 1)%, 'LineWidth',2) % Plot vectors
% quiver(Xq,Yq,Uq,Vq)
title(['Averaged Velocities and Streamlines'])
% Plot probe walls
% Plot streamlines
xlabel('x (mm)')
ylabel('y (mm)')
startx = -10:0.4:6 ;
starty = -20*ones(size(startx ));
hgridinterp = streamline(Xq,Yq,Uq,Vq,startx,starty );

Accepted Answer

Scott MacKenzie
Scott MacKenzie on 22 Jun 2021
Edited: Scott MacKenzie on 22 Jun 2021
Just adding this to the end of your code achieves what you are after (I think). The x-length of the line easy enough to calculate, or am I missing something?
% test coordinates for line, as per photo in question
xx = [-3.5 -1.75];
yy = [-17 -19];
% draw line
line(xx, yy, 'linewidth', 5);
% compute x-length
xlength = abs(min(xx)-max(xx));
% display x-length
text(-3, -16, sprintf('x-length=%.2f', xlength), 'backgroundcolor', 'w');
Addison Collins
Addison Collins on 22 Jun 2021
Ah, I actually didn't think of the brush tool! It will get the job done.

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