Question

I want to draw a function f given as 2 vectors: x and y, so y=f(x).
I use plot(x,y), but now I need it be blue above 0 and red under so it look like this:

Answer 1

When you plot a vector in matlab, any segment of that vector that is filled with NaNs will not be plotted. So one trick to accomplish your goal is to plot a second line on top of the original one with the relevant segments removed using Nans. For example,

x = linspace(1,100,1000);
y = sin(x);

% Using a cutoff of y>=0.5
belowCutoff       = y;
belowCutoff(y>=0) = NaN;  % Replace points above cutoff with NaNs; 

figure;
plot(x,y,'r',x, belowCutoff, 'b');