can't find line of best fit from simple code

I can't get a line of best fit to my data using the code from the matlab polyfit page. Does anyone know what might be going wrong?

power=[70,56,42,28];

depth90_09 =[2.9000, 3.0000, 4.4000, 1.7000];

Fit = polyfit(power,depth90_09,2);

plot(polyval(Fit,power))

hold on

scatter(power,depth90_09)

Cheers!

Answer 1

[Star Strider]

The first argument to plot must be the independent variable vector. When I added that, it works!

(I broke out the polyval call as a separate assignment, for clarity.)

power=[70,56,42,28];
depth90_09 =[2.9000, 3.0000, 4.4000, 1.7000];
Fit = polyfit(power,depth90_09,2); 
Val = polyval(Fit, power);

figure
plot(power, Val)
hold on 
scatter(power,depth90_09, 'filled')

.

[Em]

Thanks a lot! This looks great.

Is there a way to make the line of best fit continuous?

[Star Strider]

As always, my pleasure!

Thank you!

Yes! If by ‘continuous’, you intend ‘smooth’, use linspace —

power=[70,56,42,28];
depth90_09 =[2.9000, 3.0000, 4.4000, 1.7000];
Fit = polyfit(power,depth90_09,2); 
power_v = linspace(min(power), max(power), 150);
Val = polyval(Fit, power_v);

figure
plot(power_v, Val)
hold on 
scatter(power,depth90_09, 'filled')

.

[Star Strider]

As always, my pleasure!

[Em]

Thank you for all your help Star Strider! For some reason my lines of best fit don't seem to be accurate for some of the data sets. Do you know why that might be?

Here is my code:

power=[70,56,42,28];
depth90_09 =[2.9000, 3.0000, 4.4000, 1.7000];
depth50_09 =[4.3,3.4,1.8,1.4];
depth50_08 =[3,2.3,1.2,1.5];
depth50_07 =[3.3,3.3,1.2,1.6];
depth50_06 =[4,2.6,2.5,0.5];
depth50_05 =[2.7,2.7,1.8,1];
depth50_04 =[3.5,2.3,1.1,0.7];
depth50_03 =[2.8,1.4,0.7,1];
depth50_02 =[0.6,1.1,0.7,0];
power_v = linspace(min(power), max(power), 150);
Fit = polyfit(power,depth90_09,2); 
Valdepth90_09 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth90_09)
hold on 
scatter(power,depth90_09, 'filled')
%%%%%%
Fit = polyfit(power,depth50mms_09,2); 
Valdepth50mms_09 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_09)
hold on 
scatter(power,depth50_09, 'filled')
%%
Fit = polyfit(power,depth50mms_08,2); 
Valdepth50mms_08 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_08)
hold on 
scatter(power,depth50_08, 'filled')
%%
Fit = polyfit(power,depth50mms_07,2); 
Valdepth50mms_07 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_07)
hold on 
scatter(power,depth50_07, 'filled')
%%
Fit = polyfit(power,depth50mms_06,2); 
Valdepth50mms_06 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_06)
hold on 
scatter(power,depth50_06, 'filled')
%%
Fit = polyfit(power,depth50mms_05,2); 
Valdepth50mms_05 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_05)
hold on 
scatter(power,depth50_05, 'filled')
%%
Fit = polyfit(power,depth50mms_04,2); 
Valdepth50mms_04 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_04)
hold on 
scatter(power,depth50_04, 'filled')
%%
Fit = polyfit(power,depth50mms_03,2); 
Valdepth50mms_03 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_03)
hold on 
scatter(power,depth50_03, 'filled')
%%
Fit = polyfit(power,depth50mms_02,2); 
Valdepth50mms_02 = polyval(Fit, power_v);
figure
plot(power_v, Valdepth50mms_02)
hold on 
scatter(power,depth50_02, 'filled')

Cheers

[Image Analyst]

@Em your code throws an error. What is the value of

depth50mms_09

Are you sure you want a fit and not an interpolation? The quadratic fit looks reasonable to me.