MATLAB: How to determine coordinate from graph

graph coordinate

i plotted a graph.

>> x = 0:1:20; 
>> y = (668.061./x).*[1-exp(-0.1468.*x)]-40;
>> plot(x,y)

And i wanna determine the coordinate of x when y=0. (i want it be labelled on the graph). wht code should i put in?

Best Answer

Try this:

x = 0:1:20; 
y = (668.061./x).*[1-exp(-0.1468.*x)]-40;

Try this:

x = 0:1:20; 
y = (668.061./x).*[1-exp(-0.1468.*x)]-40;
Lv = ~isnan(y);                                     % Avoid ‘NaN’ Values
yq = 0;
xq = interp1(y(Lv), x(Lv), yq);                     % Interpolate To Find 'x' Value 'xq' Corresponding to 'yq'
figure
plot(x,y)
hold on
plot(xq, 0, 'r+')
hold off

It works here because ‘y’ is monotonic (not oscillating). Other approaches would be necessary otherwise.

Related Solutions

MATLAB: How to do cubic interpolation from given sets of data

The interp1 function with the 'pchip' method is the easiest way to do this (for me, anyway).

From the documentation:

'pchip' Shape-preserving piecewise cubic interpolation.

The Code —

x=[0 1 8 12 27];
y=[1 2 3 4 5];
xi = linspace(min(x), max(x));
yi = interp1(x, y, xi, 'pchip');
figure(1)
plot(xi, yi)
hold on
plot(x, y, 'pr', 'MarkerFaceColor','g')
hold off
grid

MATLAB: Getting data from plot

I agree with Jan - it's ambiguous. You already have x, and y since you plotted it, so there's no need to extract anything from the axes (graph) at all. So in that case, I'd just use the x and y which are already available, and if the x is in the x array that you plotted, you can do this:

index = find(x == desiredXValue);   % May be multiple indexes, possibly
yDesired = y(index);

Now, if the desired x is not in your x array, then you can use interp1() to get the interpolated/estimated y value for that x.

yDesired = interp1(x,y, desiredXValue);

Best Answer

Related Solutions

MATLAB: How to do cubic interpolation from given sets of data

MATLAB: Getting data from plot

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