MATLAB Answers

## add circles to starting and ending point of a line

Asked by Locks

### Locks (view profile)

on 17 Nov 2014
Latest activity Commented on by Locks

### Locks (view profile)

on 18 Nov 2014
Accepted Answer by Siam

### Siam (view profile)

Hi,
I am looking for a way to add a circle at both Ends of a line, as done manually in the Picture shown in the Appendix.
In Addition, I would also like to add such a circle in the middle of the line (at x=9) for both the cruve and the staigth line, in order to Show the convexity and therefore the value of the Point on the line being larger than the one on the curve

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## 2 Answers

Answer by Siam

### Siam (view profile)

on 17 Nov 2014
Accepted Answer

x = [1 17];
y= [19.99 18.57];
line(x,y,'Color','r','LineWidth',4)
radius = 1;
centerX = 1;
centerY = 19.99;
rectangle('Position',[centerX - radius, centerY - radius, radius*2, radius*2],...
'Curvature',[1,1],...
'FaceColor','r');
axis square;
centerX = (1+17)/2;
centerY = (19.9+18.57)/2;
rectangle('Position',[centerX - radius, centerY - radius, radius*2, radius*2],...
'Curvature',[1,1],...
'FaceColor','r');
axis square;
radius = 1;
centerX = 17;
centerY = 18.57;
rectangle('Position',[centerX - radius, centerY - radius, radius*2, radius*2],...
'Curvature',[1,1],...
'FaceColor','r');
axis square;

Locks

### Locks (view profile)

on 18 Nov 2014
thanks, this is working perfectly.

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Answer by David Young

### David Young (view profile)

on 17 Nov 2014

Here is some test data, to make a plot that looks a bit like yours:
x = 2:0.1:18;
y1 = 20 - 0.05 * x;
y2 = y1 - 0.2 * sin((x-2)*pi/16);
and this draws the line graph, with the colour specified:
plot(x, y1, 'k-', x, y2, 'b-');
Now we need the index of the arrays that corresponds to x=9. Here is one way - it finds the index for the nearest point to x=9. A different approach may be needed for your data, depending on how it is represented:
[~, index9] = min(abs(x-9));
We want circles at the start point, end point and x=9 points, so we can make an array of these indexes:
indexCircle = [1 index9 length(x)];
Finally, we can use plot to draw the circles where they are needed, with the appropriate colours. Note that three circles are drawn for each graph, but of course at the ends the pairs of circles are superimposed. We need to switch hold on to keep the old graph:
hold on;
plot(x(indexCircle), y1(indexCircle), 'ko', x(indexCircle), y2(indexCircle), 'bo');
hold off;

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David Young

### David Young (view profile)

on 17 Nov 2014
Yes, you can still use the plot function to put in the circles.
For the circle on the line you'll need to compute the correct value of y rather than indexing into an array, but that should be easy.
In general it is preferable to use the higher-level plot function rather than the lower-level line function, but in this case it makes little difference.
Locks

### Locks (view profile)

on 17 Nov 2014
sorry to bother you but could you tell me what I Need to adjust in the code so it's working also with line?
David Young

### David Young (view profile)

on 17 Nov 2014
You use line to draw the line plots, so that replaces the first call to plot in my code. You then draw the circles using the second call to plot, or something similar.

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