Plot polyfit R-squared

Hi guys,

could you please help me figure out why the polyfit works in one figure but not in the other? I don'T understand why it's not plotting the full line in the second figure.

Thank you!

load workspace_23_08_19

coeffs = polyfit(d_rel_mM, dil_rel, 1);
fittedX = linspace(min(d_rel_mM), max(d_rel_mM), 200);
fittedY = polyval(coeffs, fittedX);

coeffs_a = polyfit(d_rel_mM, time_a, 1);
fittedXA = linspace(min(time_a), max(time_a), 200);
fittedYA = polyval(coeffs_a, fittedXA);

figure(1)

plot(d_rel_mM,dil_rel,'x','Markersize',15, 'Linewidth',6)
xlabel('Imaging depth [\mum]', 'Fontname','LM Roman 10','Fontsize', 40)
ylabel('Peak dilation [%]','Fontname','LM Roman 10','Fontsize', 40)
set(gca,'Fontname','LM Roman 10','FontSize',20,'box','off')
axis tight
hold on;
plot(fittedX, fittedY, 'k-', 'LineWidth', 2);
%%
figure(3)    %% INCOMPLETE LINE, SEE ATTACHED

plot(d_rel_mM,time_a,'x','Markersize',15, 'Linewidth',6)
xlabel('Imaging depth [\mum]', 'Fontname','LM Roman 10','Fontsize', 40)
ylabel('Time constant 1/\alpha [s]','Fontname','LM Roman 10','Fontsize', 40)
set(gca,'Fontname','LM Roman 10','FontSize',20,'box','off')
axis tight
hold on;
plot(fittedXA, fittedYA, 'k-', 'LineWidth', 3);

Answer 1

[dpb]

Because

>> fittedXA(1),fittedXA(end)
ans =
    10
ans =
    20
>>

and that's what you plotted as the abscissa for some reason whereas the data are plotted versus ImagingDepth which has range of

>> min(d_rel_mM),max(d_rel_mM)
ans =
     0
ans =
  345.4000
>> 

You've mixed up x and y for the prediction line evaluation...use

coeffs_a = polyfit(d_rel_mM, time_a, 1);
fittedXA = linspace(min(d_rel_mM), max(d_rel_mM), 200);
fittedYA = polyval(coeffs_a, fittedXA);

and all will be well...

[gorilla3]

Ahhh wonderful! Thank you :)

[dpb]

Of course, you DO realize that since is a first order fit, you only need two points to draw the line, right? :)