Trouble Plotting Multiple Data Sets on Same Graph

UPDATE: I needed to convert my density (rho) from g/cm^3 to kg/m^3 to become significant. Thanks for the help, all.

I'm trying to plot a data set and an equation on the same figure, and for some reason the equation keeps coming up as just a straight line. When I separate the two, though, the equation plots perfectly. Can someone help me, please? Here's my code:

if true
  % mew=0.8; % 0.6 to 0.1
lam=0.9; % 0.4 to 0.9
rho=2.7; % average crustal density in g/cm^3
D=12; % 15 km to 10 km
x=(-20e3:100:20e3); % in m
V=9.5129e-10; % 3 cm/yr into m/s
tau=mew/(1+mew^2)*(1-lam)*rho*9.8*D;  % from equation tau = u/(1+u^2) * omega
q=1e3*tau*V/pi*(1-x./D.*atan(D./x)); % in mW/m2
Obs=[
-18.2   78.892
-13.8   73.071
-11.2   74.65
-4.672  76.751
-4.262  75.056
-3.996  81.076
-3.1    83.86
-2.843  76.377
-0.165  76.931
0.267   82.343
0.684   83.022
0.736   82.221
0.893   78.375
1.732   75.687
1.785   75.569
3.785   78.503
4.155   74.037];
distance=Obs(:,1);  % in km
heat=Obs(:,2);  % in mW/m2
figure(1)
plot(x/1e3, q+73,'b', distance, heat, 'ko')
title ('SAF Heat Flow ') 
xlabel ('Distance From Fault (km)') 
ylabel('Heat Flow (mW/m^2)')
legend (['\mu = ', num2str(mew),' \lambda = ', num2str(lam), ' D= ',num2str(D/1e3)])

end

[Jonathan Chin]

Look at the scale of q, it is very small. The data just looks flat try.

    ylim([73,73.00001])

You could try scaling q to see more variation visually

Answer 1

[Alison Richards]

I figured it out!! My density was in g/cm^3, but when I manipulated it to kg/m^3, it starts to better fit the curve. Thank you all for your help, I really appreciate it.

Answer 2

[Geoff Hayes]

Alison - do you mean that

 plot(x/1e3, q+73,'b')

plots perfectly? If I run your code, then yes, there is a spike at zero so I don't see a flat line. But you need to consider your q data. The minimum value is 5.6281e-13 and the maximum value is 4.6901e-06. These are very small numbers and when you add them to 73 then you are plotting values that are all close to 73.

When you plot just the above, then the y-axis ranges over such a small interval (look closely at it) that the spike at zero can easily be seen. But when you plot this with the other data, you don't get that granularity and so the spike is flattened. You would have to zoom in to see it.

Perhaps there is a problem with the q data. Can you confirm that it is as you expect?

[Alison Richards]

I believe this is correct--my goal is to manipulate mew and lambda to fit the data provided, but i'm struggling as I'm not able to see the peak because the q values are so small/close to 73.

[Geoff Hayes]

You could try plotting your data using two y-axes. See plotting with two y-axes for an example.

Or if you have an older version of MATLAB (like mine which does not have yyaxis), you can try using plotyy like

[hAxes] = plotyy(x/1e3, q+73, distance, heat)
% change the lifestyle for the second line 
hLine2 = get(hAxes(2), 'Children');
set(hLine2,'LineStyle','o')

The above makes some assumptions (that there is at least two axes handles, and that there is only one child) so you may want to make some changes to make the code more robust.