Help-Plot of two vectors - scatter plot with two subsamples??

2012 5 13
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Hi all,
I have the following question:
I have two column vectors X (min value= -0.5; max value= 1.1; 1000 observations) Y (min value =-0.7; max value= 3; 1000 observations)
I would like to perform the following plot. Plot a subset of X (for -0.1 to 0.5) on the x-axis and the corresponding observations of Y on the y-axis.
I would like to split the Y observations in two groups around the point X=0, such that I have observations to the left of X=0 and to the right of X=0.I would also like to fit regression lines (lines of best fit) to the left and right of X=0 and compare their slopes. Can anybody help me with this difficult challenge?
Thanks for your time!

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Walter Roberson
Walter Roberson 2012-5-13

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availcolors = [1 0 0; 0 1 0]; %arbitrary, red and green
colidx = 1 + (X >= 0); %1 on the left, 2 on the right
colorarray = availcolors(colidx,:); %array of rgb
pointsize = 10; %arbitrary
scatter(X, Y, pointsize, colorarray)

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Jarek
Jarek 2012-5-13

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Thank you Walter Roberson for taking the time to look through my problem.
That is great!
Still, two issues remaining:
1. How to plot a subset of X (for -0.1 to 0.5) on the x-axis and the corresponding observations of Y on the y-axis.
2. I would also like to fit regression lines (lines of best fit) to the left and right of X=0 and compare their slopes.
It would be great if somebody could help me on this?
Thanks for your time and effort!

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Walter Roberson
Walter Roberson 2012-5-13
in_subset = (X >= -0.1 & X <= 0.5);
X = X(in_subset);
Y = Y(in_subset);
Regression lines you should now be able to figure out for yourself using logical indexing.
Jarek
Jarek 2012-5-13
Thank you Walter Roberson for the swift reply, this is really cool!!
As you can see, I am completely new to matlab.
I have found the Basic Fitting sub-menu in Tools-menu, however it fits only one line for the whole sample, and I need two separate lines for X=[-0.1 to 0] and [0 to 0.5].
Unfortunately, I couldn’t figure out how this logical indexing works, still trying though.
It could be great if you could help on this!
Thanks again for your time and effort!

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