How do I find the instersection on my plot?

I have a plot with the errorbar. The data coming from a matrix 21x6 and the errorbar from another matrix. I need to find where (for a given y-axis value) the functions intersect the curves and at what coordinate. Since the values of the graph are given by a matrix, only the y axis correspond to the physical values, for the x axis I changed the values via

xticks([1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21])
xticklabels({'-20','-18','-16', '-14', '-12', '-10', '-8', '-6','-4','-2','0','2', '4', '6', '8', '10', '12', '14', '16', '18', '20'})

The graph is this one:

Answer 1

[Matt J]

You can use fzero, e.g.,

fun=@(x)x^2;
y=4;
x_intersect=fzero(@(x) fun(x)-y,[0,1e6])

x_intersect = 2.0000

[Matt J]

If you only have discrete samples of the curves, you will have to select an interpolation model, e.g.

xdata=(0:0.3:10);  ydata=xdata.^2; %plotted samples

fun=griddedInterpolant(xdata,ydata,'linear'); %interpolator

y=4; %given y value

x_intersect=fzero(@(x) fun(x)-y,[0,1e6])

x_intersect = 1.9949

[HelpAStudent]

I don't understand

xdata=(0:0.3:10);
ydata=xdata.^2;

Can you explain?

[Matt J]

It's just example plot data.

[HelpAStudent]

Ok, I think you does not understand my problem. My plot is a matrix. I have 6 curves on the plot because the matrix it's 21x6. A curve was drawn from the 21 values ​​identified. For a total of 6 curves with 21 values ​​each (the dots in the graph). If you see the graph I know a value of the yaxis but it does not correspond on something because the plot is made of the matrix

[Matt J]

You Accept-clicked the answer so I assume you figured it out? If not, I don't see how my answer is invalid. If you have 6 curves, just apply my technique 6 times, or use a loop.

[HelpAStudent]

Your answer it's not ok, because you don't use the right function: you used fun=@(x)x^2; but I have a matrix, so I don't know the equation for my function. I just wanna know HOW from the graph that I share with you I can find where for y=80 the courves interpolate. The courves are the columns of the matrix and the rows are the point (the dots on the plot)

[Matt J]

I already showed you how to modify the technique when you only have discrete plot data xdata and ydata. Here it is again.

xdata=(0:0.3:10);  ydata=xdata.^2; %plotted samples

fun=griddedInterpolant(xdata,ydata,'linear'); %interpolator

y=4; %given y value

x_intersect=fzero(@(x) fun(x)-y,[0,1e6])

[HelpAStudent]

Yes, but the y data are not the power of the x data as you write. And I can't give an y value because I have just the matrix of the value. I did not create a plot from x and y values, I create a plot from the value and matlab create somehow the value of the x axis and y axis, but I have them only on the graph, not as a variable in my script

[Matt J]

The ydata would be a particular column in your matrix, which you do have. The xdata, based on what you posted, would just be,

xdata=-20:2:20

xdata = 1×21

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

In other words, you could have (and probably should have) plotted the data as follows, rather than manipulating the xticklabels.

plot(xdata,Matrix)

[HelpAStudent]

I do not have ydata as the columns of my matrix. As you can see:

meanbasketT = transpose(meanbasket);
y = meanbasketT;
e = transpose(devstdbasket);
figure; errorbar(y,e,'-o')
legend({'Function 1', 'Function 2', 'Function 3', 'Function 4', 'Function 5', 'Function 6'}, 'Location', 'northwest', 'FontSize',14 );
xticks([1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21])
xticklabels({'-20','-18','-16', '-14', '-12', '-10', '-8', '-6','-4','-2','0','2', '4', '6', '8', '10', '12', '14', '16', '18', '20'})

[HelpAStudent]

The meanbasketT is 21x6 that's why I have 6 function and 21 dots for function

[Matt J]

The ydata is not a particular column. The column as I told you before are the different line

Yes, the ydata in my original example also represented one particular line, but as I told you you can loop over your matrix columns and apply my technique to each.

xdata=-20:2:20
for i=1:6
 ydata=Matrix(:,i);
 
 y=...
     
 fun=griddedInterpolant(xdata,ydata,'linear'); %interpolator 
 
 x_intersect(i) = fzero(@(x) fun(x)-y,[0,1e6])
    
end