How to set background in multiple colors in semilogy plot (MATLAB) ?

Question

I am looking for a way to shade the background of my semilog plot in two colors.
For example, in the following image, I have am plotting three polynomials and they all are equal at x=1. I want one rectangle for x<1 region and other for x>1 region. How can I insert two such rectangles, of different colors, in background to highlight these two regions.

MWE:

   x = 0.1:0.1:10;
   y1 = polyval([1, 0], x);      % Evaluate y = x;
   y2 = polyval([1, 0, 0], x);   % Evaluate y = x^2;
   y3 = polyval([1, 0, 0, 0], x);   % Evaluate y = x^3;    
   figure
   semilogy(x, y1, '.k', x, y2, '.b', x, y3, '.r');    title ('Three
   polynomials on a semilog y scale')    xlabel('x');    ylabel('y');   
   legend({'y= x', 'y = x^2', 'y = x^3'}, 'Location', 'Northwest')

Check these posts: [Transparent background for identification of 0 and 1 in a MATLAB figure](https://stackoverflow.com/questions/46527808/transparent-background-for-identification-of-0-and-1-in-a-matlab-figure) and [Highlight parts of matlab plot](https://stackoverflow.com/questions/13734086/highlight-parts-of-matlab-plot) — Sardar Usama, Oct 05 '17 at 17:55
@SardarUsama Sorry, i typed the answer before seeing your comment. Indeed I follow the same path to solve, however when i searched, it was nothing with *logscale area*. — Guto, Oct 05 '17 at 18:00

Guto · Accepted Answer · 2017-10-05T19:27:59.660

1

You can solve that using area or patch.

As pointed by @SardarUsama, there are others questions with good examples on it, however, you need to avoid to have any zeros in the area data, otherwise it will fail.

Follows the code setting one area only.

x = 0.1:0.1:10;
y1 = polyval([1, 0], x);      % Evaluate y = x;
y2 = polyval([1, 0, 0], x);   % Evaluate y = x^2;
y3 = polyval([1, 0, 0, 0], x);   % Evaluate y = x^3;    
figure
plot(x, y1, '.k', x, y2, '.b', x, y3, '.r'); %MODIFIED   
hold on  %ADDED
title ('Three  polynomials on a semilog y scale')   
set (gca, 'Yscale', 'log'); %ADDED
xlabel('x');   
ylabel('y');   
legend({'y= x', 'y = x^2', 'y = x^3'}, 'Location', 'Northwest')

area( [1 1 10 10],[1e-3 1e+3 1e+3 1e-3 ],'FaceColor','green','facealpha',0.3)  %ADDED

The code above works for matlab after 2014b. If you have one before that, you can use the patch function (which requires some small change in the data, but uses the Facealpha option) or you can move the area to the background as i do below:

ax=get(gca,'Children'); %ADDED
set(gca,'Children',[ax(2) ax(3) ax(4) ax(1)]); %ADDED, move area to background

Note: Indeed, I missed the problem with the legend. I correct as mentioned, however for me the area was on top of the other graphs. To solve it i changed the order of the plots. If the area was with transparency, this will not be an issue.

edited Oct 05 '17 at 19:27

answered Oct 05 '17 at 17:58

Guto

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Thank you. That seems to work except the first two entries in legend are showing up as rectangles instead of lines. – NAASI Oct 05 '17 at 18:29
@Guto use `area` after you're done plotting and with the `FaceAlpha` property. i.e ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍ ‍‍‍‍‍‍ ‍‍‍‍‍‍ ‍‍`area([1,1,10,10],[1e-3,1e+3,1e+3,1e-3],'FaceColor','green','FaceAlpha',0.3)` – Sardar Usama Oct 05 '17 at 19:20
@SardarUsama Thanks, I'm using 2014b, so it does not work the `facealpha`. I edited the question considering both solutions. – Guto Oct 05 '17 at 19:30
@NAASI Indeed i missed, I correct with your idea and extra options to solve the transparency issue as well. – Guto Oct 05 '17 at 19:35

How to set background in multiple colors in semilogy plot (MATLAB) ?

1 Answers1