How to plot Multiple trend-lines on one scatter plot

Hi every one,

I have a scatter plot and I need to do a second order trend line for some part of my data (just for the values less than 15 or 10 on x axis). Do you know how can I do that?

Answer 1

[KSSV]

You can pick those respective points and fit a curve to plot the trend. Let x,y be your points.

x1 = x(x<=15) ;  % pick points less than 15 
y1 = y(x<=15) ; 

p1 = polyfit(x1,y1,1) ;  % Fit a line 

figure
hold on
plot(x,y,'.k')
plot(x1,polyval(p1,x1),'r')

[Nicky T]

Thank you very much. Do you know how can I connect my 2nd order trendine to the horizontal line (where y=100)? Infact I do not know what is the exact change point. I want my trand line to be connected to the horizontal line for (x>15 or whatever is the change point) please see attached

[KSSV]

Same thin, pick those points and fit a curve again.

x2 = x(x>15) ; 
y2 = y(x>15) ; 
p2 = polyfit(x2,y2,1) ; 

Note: I have edited the answer, there were few typo errors.

[Nicky T]

Please see the plot below. Do you know how can I merge all three trendlines in one trendline that covers all the areas?

[KSSV]

You have to take continuous x-data to join them. In here you have taken different values. Show the code which you tried.

[Image Analyst]

I think you meant 2 in polyfit(), and p1 in polyval().

p1 = polyfit(x1,y1,2) ;  % Fit a second order polynomial 
figure
hold on
plot(x,y,'.k')
plot(x1,polyval(p1,x1),'r')  % p1, not p

[KSSV]

@Image Analyst..yes..corrected it.

[Nicky T]

here is the code, it plots fitted line or curve for each part but does not connect them:

x1 = tOut(tOut>tRet) ;  % pick points for min req ventilation mode
y1 = Sodamp1(tOut>tRet);
x2 = tOut(tOut<tSupp) ;  % pick points for economizing mode
y2 = Sodamp1(tOut<tSupp) ;
x3 = tOut(tOut<tRet & tOut>tSupp); % pick points for economizing+cooling mode
y3 = Sodamp1(tOut<tRet & tOut>tSupp);
p1 = polyfit(x1,y1,1) ;  % Fit a line 
p2 = polyfit(x2,y2,2)    % Fit second order or exponential ?
p3 = polyfit(x3,y3,1) ;
figure
ylim ([0,100]);
hold on
scatter(tOut,Sodamp1,'.c')
plot(x1,polyval(p1,x1),'.k')
plot(x2,polyval(p2,x2),'.k')
plot(x3,polyval(p3,x3),'.k')

[Image Analyst]

Nicky, please draw what you want in red over your scatterplot. And since you seem to have three separate trends (2 flat and one curved), why do you want their endpoints connected?

[Nicky T]

How do I draw what I want with Matlab? It is the ideal behavior of my damper in 3 different modes. I want them to be connected as one trendline and then show them with the measured data in one graph

[KSSV]

What are values of tRet, tSupp ??

[Nicky T]

those are temperature. I have an excel file with lots of data.