Shade the region between a curve and the vertical axis

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Nathaniel H Werner 2020-1-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/499138-shade-the-region-between-a-curve-and-the-vertical-axis

评论： Star Strider 2020-1-8

I have several velocity profiles in a figure, and I've been trying to play around with the area function to try and shade the region between the curve a vertical line. I'm looking to create a plot like this.

Screen%20Shot%202014-03-16%20at%208.45.32%20PM.png

Here is the relavent parts of my code.

% V is the dependent variable, plotted on the x-axis
% Y is the independent variable, plotted on the y-axis
% My actual data is not attached but you can use tan(x) -1<x<1 or tan^-1(x) mirrored about y = x
Y = linspace(-1,1,50);
V = atan(Y);
% R is an array for the radial positions along the wing, the zero location has to move with each radial position
R = 1:4;    % for this case
% plot data
plot(V+(R(1)),Y);    % only using the first R value for this example
% draw vertical line for reference
plot(R(1)*ones(1,50),Y,'k')

Now if I try to shade in the region, let's say from a base of 0 this is the result.

area(V+R(1),Y,1,'facealpha',0.5)

As expected the area function fills in the region from Y = 1 to V. I tried switching X and Y as some have suggested, but I don't get the results I would like. The area function does what's it's supposed to and shades in the region up to V = atan(X)+1 from Y = 1.

I would like to shade along the X axis (instead of Y) up to the curve.

This last figure shows what I am actually trying to accomplish but with my actual data instead of with V = atan(Y) as I did above.

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Answer 1

Star Strider 2020-1-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/499138-shade-the-region-between-a-curve-and-the-vertical-axis#answer_408913

编辑：Star Strider 2020-1-6

在 MATLAB Online 中打开

I am not certain what you want.

Try this:

Y = linspace(-1,1,50);
V = atan(Y);
% R is an array for the radial positions along the wing, the zero location has to move with each radial position
R = 1:4;    % for this case
% plot data
plot(V+(R(1)),Y);    % only using the first R value for this example
% draw vertical line for reference
hold on
plot(R(1)*ones(1,50),Y,'k')
Vidx = (V+R(1)) > 1;
Vones = ones(size(V));
patch([V(Vidx)+R(1) fliplr(V(Vidx)+R(1))], [Y(Vidx) Vones(Vidx)], [0.1 0.3 0.5], 'FaceAlpha',0.3)   % ‘patch’ Call
hold off

It uses the patch function to define the area I believe you want to shade. Increase the number of elements in ‘Y’ to increase the resolution of the patch object.

EDIT —

Added plot image —

1Shade the region between a curve and the vertical axis - 2020 01 06.png

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Nathaniel H Werner 2020-1-7

在 MATLAB Online 中打开

No I don't think this is what I am looking for. But I understand what you did and I think I may be able to work with it.

I can define the region I want to be shaded like this. I apologize if I was unclear in the original question. Recall the x-axis is the dependent variable and it "restarts" from zero at each black line, so I will be calling it the V-axis. The y-axis is the independent variable but actually represents the vertical height so I will still call it the y-axis.

(y-axis)

(x-axis, 0 restarts at each black line which is why I use V+R instead of V)

You can see why the code you provided does not work if I make the following changes.

Yidx = (Y>-0.4 & Y<0.6);
patch([V(Yidx)+R(1) fliplr(V(Yidx)+R(1))], [Y(Yidx) Vones(Yidx)], [0.6 0.5 0.2], 'FaceAlpha',0.3)   % ‘patch’ Call

If I limit the values in y-axis between -0.4 and 0.6 I get the second region. The y-axis should be at the black line at 1, because that is where V = 0. The blue area I've filled in by hand corresponds to what I am looking for in this case. In this case V(y) has a region on the left and right of 1.

Star Strider 2020-1-7

在 MATLAB Online 中打开

The initial description was confusing.

Try this:

Y = linspace(-1,1,50);
V = atan(Y);
% R is an array for the radial positions along the wing, the zero location has to move with each radial position
R = 1:4;    % for this case
% plot data
plot(V+(R(1)),Y);    % only using the first R value for this example
% draw vertical line for reference
hold on
plot(R(1)*ones(1,50),Y,'k')
Vidx1 = ((V+R(1)) >= 1) & (Y <= 0.6);
Vidx2 = ((V+R(1)) < 1) & (Y >= -0.4);
Vones = ones(size(V));
patch([V(Vidx1)+R(1) fliplr(V(Vidx1)+R(1))], [Y(Vidx1) Vones(Vidx1)*0.6], [0.1 0.3 0.5], 'FaceAlpha',0.3)       % ‘patch’ Call
patch([V(Vidx2)+R(1) fliplr(V(Vidx2)+R(1))], [Y(Vidx2) Vones(Vidx2)*(-0.4)], [0.5 0.3 0.5], 'FaceAlpha',0.3)    % ‘patch’ Call
plot(xlim, [1 1]*0.6, '--r')                                                                                    % Dashed Horizontal Red Line
plot(xlim, [1 1]*(-0.4), '--r')                                                                                 % Dashed Horizontal Red Line
hold off

That appears to be closer to what you just now described:

1Shade the region between a curve and the vertical axis (2) - 2020 01 06.png

The patch calls are straightforward. The key to defining them are ‘Vidx1’ and ‘Vidx2’, both being ‘logical index’ vectors. To understand how they work, see the documentation on Matrix Indexing.

I am not certain what the dashed red lines indicate. I added them here. To delete them, just eliminate the two plot calls that create them.

Nathaniel H Werner 2020-1-8

That makes sense. I'll provide the solution I found as an answer later when I have time. Thank you for your help, and I apologize for the confusion. This is a surprisingly difficult problem.

Star Strider 2020-1-8

As always, my pleasure.

While I would like to see the actual problem, itwould be best to post a detailed description of the problem — and your solution (and all other necessary code) — as a new Question if you want specific help with it.

It would be best to add a Comment with a link to the new Question here, for continuity.

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Shade the region between a curve and the vertical axis

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