# plot different color

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Steven on 16 Nov 2011
Hi,
Let's say I have x = 1:100; y = sin(x); I wish to plot it in blue but if y is superior at 0.5, I want that the plot changes color for red.
Thx

Kelly Kearney on 16 Nov 2011
Another way of doing this would be to use a patch object instead of a line; that way you can get a color-change effect with only one object, and don't need to figure out the change points manually.
x = 0:pi/20:10*pi;
y = sin(x);
c = sign(y - 0.5);
p = patch([x NaN], [y NaN], [c NaN], [c NaN], 'edgecolor', 'interp');
colormap([0 0 1; 1 0 0]);
Note that you may need to increase the number of points defining your line in order to get a relatively narrow color transition zone (for example, in your original example with x = 1:100, the coarseness of the plot is not ideal for this technique).

Sven on 16 Nov 2011
Steven, try this, it plots two separate plots depending on whether y is above/below 0.5:
x = 0:pi/20:10*pi; y = sin(x);
Now, this plot kind of does what you said, but not what (I think) you intend. I think that you want the lines joined, and you want the line to change colour every time it crosses the y=0.5 axis. That's a little more tricky. We will need to first calculate the X-location of every crossing of the y=0.5 line using Doug Schwarz's very good intersections submission
[Xx,Yx Xinds] = intersections(x,y,x([1 end]),[0.5 0.5]);
We then make segments between each of the crossings:
xSegs = arrayfun(@(from,to)[Xx(from) x(ceil(Xinds(from)):floor(Xinds(to))) Xx(to)], 1:numXsecs-1, 2:numXsecs,'UniformOutput',false);
ySegs = arrayfun(@(from,to)[Yx(from) y(ceil(Xinds(from)):floor(Xinds(to))) Yx(to)], 1:numXsecs-1, 2:numXsecs,'UniformOutput',false);
Next we append the start and end segments:
xSegs = [{[x(1:floor(Xinds(1))) Xx(1)]} xSegs {[Xx(end) x(ceil(Xinds(end)):end)]}];
ySegs = [{[y(1:floor(Xinds(1))) Yx(1)]} ySegs {[Yx(end) y(ceil(Xinds(end)):end)]}];
Now we ask which of these segments should be red or blue?
Now we plot each segment separately:
figure, hold on
Wow. That was more complicated than I expected. I'm sure there are different ways to do this. Note that I've included lots of cellfun() and arrayfun() code... if you're not familiar with those functions they can be a little daunting. Hope this helped.