# How to plot a second graph instead of color-coding

5 views (last 30 days)
IceQueen on 26 Dec 2012
Hello everyone,
i just started with my master thesis and i already am in trouble with my capability/understanding of matlab.
The thing is, i have a trajectory on a surface of a planet/moon whatever (a .mat with the time, and the coordinates. Then i have some .mat with time and the measurement at that time.
I am able to plot this as a color coded trajectory (using the measurement and the coordinates) in scatter(). This works awesomely nice.
However my problem is that i need something more sophisticated. I now need to take the trajectory and instead of color-coding it, i am supposed to add the graph (value) of the measurement (which is given for each point) to the trajectory (which is not always a straight line). I will added a little sketch to explain what i want. The red arrow shows what i want to add to my plot and the green shows what i have.
I would be really thankful for any kind of suggestion or help! Teja Muppirala on 28 Dec 2012
Assumung your XY data are smooth, you can find the normal vector at each point, and then offset in that direction, like the code below. If your position data is not smooth, then you'll need to smooth it before you differentiate and get the normals or otherwise you'll get a bunch of noise.
%%%%%%%%%%Step 0. Just making some random data... %%%%%%%%%%
% Position
rng(16);
x = interpft(3*rand(1,4),1000);
y = interpft(3*rand(1,4),1000);
x = x(1:200);
y = y(1:200);
% Measurement
m = 0.1*interp1(1:20,rand(1,20),linspace(1,20,200),'pchip');
% Plot it
subplot(211);
plot(x,y,'r','linewidth',2);
hold on;
plot(x(1),y(1),'rx','markersize',10);
title('XY position');
axis equal;
grid on;
subplot(212);
plot(m,'k','linewidth',2);
title('Measurement');
%%%%%%%%%%Step 1. Find the normal vectors %%%%%%%%%%
% First find the velocity using finite differences
dx = conv(x,[-0.5 0 0.5],'valid'); % Same as (x(3:end) - x(1:end-2)) / 2
dx = [dx(1) dx dx(end)]; % Take care of the endpoints
dy = conv(y,[-0.5 0 0.5],'valid');
dy = [dy(1) dy dy(end)];
% Normalize it to unit length
V = [dx; dy];
Vmag = sqrt(sum(V.^2)); % Magnitude of velocity
V1 = bsxfun(@rdivide, V, Vmag); % Velocity vector normalized to 1
% Swap the components to get the normals
N1 = [V1(2,:); -V1(1,:)];
subplot(211)
quiver(x,y,N1(1,:),N1(2,:),0.5);
%%%%%%%%%%Step 2. Shift the measurement to the new axes %%%%%%%%%%
m_shifted = [x;y] + bsxfun(@times,m,N1);
plot(m_shifted(1,:),m_shifted(2,:),'k','linewidth',2);
hold off;
legend({'XY Path', 'Start pt.', 'Normals', 'Measured'})
IceQueen on 28 Dec 2012
yes thanks. i asked this question in another forum as well and got in general the same answer. I will try it out when i have access to the data again. Thank you very much :)

Walter Roberson on 27 Dec 2012
You need to do a rotation (and possibly a translation as well) on your (x,y) data. Possibly the easiest way to do that would be to create the plot line in a hggroup and then do a hgtransform to do the rotation and translation.
IceQueen on 27 Dec 2012
i will have a look on those two functions. thanks.
edit: it might help, but i have for every point already the value.. just turning the graph around leaves me with the problem on how i would align it to the right 'point' on the trajectory (which btw will not be color coded anymore)

Lalit Patil on 27 Dec 2012
plot(data1)
hold on
plot(data2)
IceQueen on 27 Dec 2012
thanks, but thats not the problem. I work a lot with hold on (i even have an image as background, but not for the purpose of the question). The thing is that every single point has a value and i need to put that at a distance away in an right angle at that point... maybe the easiest is to somehow find the tangential of the whole grapg in several points and then take that as new x-axis, but i really don't know how to do that.