So, meshgrid generates a grid of x and grid of y values so index wise you get the xy pair-combo. usually very useful when you're building a grid of points. here is a bit of snippet of code that goes through for you the reshaping of data and meshgrid
close all
data = dlmread('run27_avg.txt',' ',1,0);
%just get the min and max of the data x and y
spanX = [min(data(:,1)) max(data(:,1))];
spanY = [min(data(:,2)) max(data(:,2))];
%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 an equally spaced grid
Uq = griddata(data(:,1),data(:,2),data(:,3),Xq,Yq);
Vq = griddata(data(:,1),data(:,2),data(:,4),Xq,Yq);
figure(1),quiver(Xq,Yq,Uq,Vq)
startx = 0.1:0.1:1;
starty = ones(size(startx));
hgridinterp = streamline(Xq,Yq,Uq,Vq,startx,starty);
set(hgridinterp,'color','green');
%if in a grid you can reshape the data (looking at the file x is listed
%first from 1:640 for 540 times... so after reshaping to 640x540(rows x col)
%transpose it so x runs along the horizontal
reshapeX = reshape(data(:,1),640,540)';
reshapeY = reshape(data(:,2),640,540)';
reshapeU = reshape(data(:,3),640,540)';
reshapeV = reshape(data(:,4),640,540)';
%
hreshape=streamline(reshapeX,reshapeY,reshapeU,reshapeV,startx,starty);
set(hreshape,'color','red')
set(hreshape,'linestyle','-.')
for you it does look like that data is in a grid but not equally spaced. at first i thought streamline would care but i guess not. the X values (data(:,1)) jumps between 0.057 and 0.0571 delta values (difference between points).
