plot different color

Question

Steven 2011-11-16

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https://ww2.mathworks.cn/matlabcentral/answers/21382-plot-different-color

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

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Answer 1

Kelly Kearney 2011-11-16

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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).

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Answer 2

Sven 2011-11-16

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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);
mask = y>0.5;
figure, plot(x(mask),y(mask),'r.', x(~mask),y(~mask),'b.')

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

http://www.mathworks.com/matlabcentral/fileexchange/11837-fast-and-robust-curve-intersections:

[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?

highMask = cellfun(@max,ySegs)>0.5;

Now we plot each segment separately:

figure, hold on
cellfun(@(x,y)plot(x,y,'b'),xSegs(~highMask),ySegs(~highMask))
cellfun(@(x,y)plot(x,y,'r'),xSegs(highMask),ySegs(highMask))

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.