How can I shade my plot to the right of a given x value?

I would like the plot area (under the plot lines) to be shaded grey to the right of x = 2.

Answer 1

[Star Strider]

Use the patch function. It takes a bit of experimenting to get it to do what you want, but it’s relatively easy.

To draw a gray box in specific axes, for example:

cm = [0 0 0 ;  0.5 0.5 0.5;  1 1 1];
figure(1)
patch([1 1 2 2 1]', [1 2 2 1 1]', cm(2,:))
axis([0 3    0 3])

[Star Strider]

@A K M Kamrul Hasan —

What you want to do is relatively straightforward —

x = linspace(0, pi, 250).';                                 % Assume Column Vectors
y = sin(x);
[ymx,ymxidx] = max(y);

Pm = 0.5;
deltac = pi/2;
delta0 = pi/4;

yidx = find(diff(sign(y - Pm)));
for k = 1:numel(yidx)
    idxrng = max(1,yidx(k)-1) : min(numel(x), yidx(k)+1);
    xval(k) = interp1(y(idxrng), x(idxrng), Pm);
    yval(k) = interp1(x(idxrng), y(idxrng), xval(k));
end

Lv1 = x >= xval(1) & x <= deltac;                           % Logical Vector (Red Area)
% Q1 = nnz(Lv1)
Lv2 = x >= deltac & x <= xval(2);                           % Logical Vector (Green Area)

figure
plot(x, y)
hold on
Pmv = ones(size(x))*Pm;
patch([x(Lv1); flip(x(Lv1))], [zeros(size(x(Lv1))); flip(Pmv(Lv1))], 'r')
patch([x(Lv2); flip(x(Lv2))], [zeros(size(y(Lv2)))+Pm; flip(y(Lv2))], 'g')
plot([0 x(ymxidx)], [1 1]*ymx, '--c', LineWidth=2)
plot([1 1]*xval(2), [0 yval(2)], '--m', LineWidth=2)
hold off
% axis('padded')
ylim([min(ylim) max(y)*1.1])
text(mean([pi/2 xval(1)]), 0.25, 'A_1')
text(0.6*pi, 0.75, 'A_2')

I leave the annotations and precise colours to you. (I colored the patches red and green and the dashed lines cyan and magenta to make them easier to distinguish in the code.)

The purpose for the interpolation is to get relatively precise values for the (x,y) coordinates of the curve intercepts with ‘Pm’. They are not absolutely necessary if simply uising the appropriate index coordinates will work.

.

Answer 2

[Geoff Hayes]

The area function (type help area in the Command Window for details) can be used to shade in a portion of your plot to the right of a given x value. Consider the following example:

 x = -4:0.1:4;
 y = x.^2;     % usual parabola

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

 % find the index of the element in x that is equal to two (works well for this
 % example, but in other cases you may have to choose the index that you want to
 % start the shading from)
 idx = find(x==2);

 % now colour the area in grey to the right of x==2
 grey  = [127 127 127]./255;
 hold on;
 area(x(idx:end),y(idx:end),'basevalue',0,'FaceColor',grey);

In the above, we use a subset of x and y to define the region that we want to shade in grey, starting at a base value of 0 (i.e. y==0).

[A K M Kamrul Hasan]

Hi,

This discussion is about the srea bounded by straight lines. However, is it possible to shade an area in Matlab plot bounded by straight lines and curved lines? (i.e like the attached image) ?

Regards,

Answer 3

[Thorsten]

x = -10:0.1:10;
f = @(x) -x.^2 + 100; % sample function
finv = @(y) sqrt(-(y - 100)); % inverse of sample function
y = f(x);

y1 = 50; % sample Pm
x1 = finv(y1);
plot(x, y, '-k')
hold on
xline(0)
yline(y1)
% construct the polyshape from its x and y values
ps = polyshape([0 x1 floor(x1):-0.1:0], [50 50 f(floor(x1):-0.1:0)]);
plot(ps, 'FaceColor', [228 133 173]/255, 'FaceAlpha', 1)