getting data from plot ?

I plot an x , y graph can i ask the programme to tell me the value of y for the value of x i want ? how i can write these at the command window ?

[Nastaran kh]

if you plot, in section tools,basic fitting you can evaluate

Answer 1

[Image Analyst]

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);

[lclk psdk]

i am so sorry for complexity the problem is that i plotted a figure of x , y then i want the program automatically uses the y value (any value) when its x value used (it is used in a loop) thanks alot for ur care

[Image Analyst]

Isn't that exactly what I just showed you how to do? In fact I showed you how to do it two different ways. Explain why my code does not do what you want.

[lclk psdk]

thank u alot ,,, i found ur answer very useful,,,,,,, i only wanted to explain my question

[George Black]

Man, I just spent so long trying to figure this out. such a simple issue, but I am a Matlab novice. Great explanation.

[jakub rolny]

Thank you so much, helped a lot :)

[Chee Xiang Chong]

thanks

[Koosha]

Thank u so much.

[DEEPIKA E]

can you please say how it would be if I want to find x value corresponding to y value using interp1? @Image Analyst

[Image Analyst]

% Find the index of yDesired closest to your target y value
[~, index] = min(abs(yDesired - yTarget))
xTarget = x(index); % Or xDesired(index) if you used an array in interp1()

[DEEPIKA E]

Thank you

[Image Analyst]

change is your y so

y = ((Temperature-Ti)/(Tf-Ti))*100

and x is time according to your plot(time, change) call. By the way, don't use time, or any other built-in function name as your variable name because it could lead to confusion and errors. And don't call one variable Time and another time - that's just downright confusing! But anyway, I can't figure out how your y depends on x. Can you give me an equation where y is a function of x? If you want Temperature as a function of change, it's simply

Temperature = (change/100) * (Tf - Ti) + Ti

Answer 2

[Jan]

The question is not clear. If you plot x versus y, the values are known and therefore the problem has not relation to the plotting. But perhaps you have a Figure file and lost the x and y values. Please explain this by editing the original questions.

Do you want the y values exactly at the position of an x value, or are you interested in x values between two given x values also? In the first case consider, that numerical comparisons are bounded by the numerically limited precision:

x = 0:0.1:1;
find(x == 0.3)  % does not find a matching element!

So perhaps this helps:

axes;
x = 0:0.1:pi;
y = sin(x);
plot(x, y);
clear('x', 'y');  % for demonstration only

LineH = get(gca, 'Children');
x = get(LineH, 'XData');
y = get(LineH, 'YData');

xx = 0.724  % Any value
yy = interp1(x, y, xx)  % Linear interpolation [EDITED, not "linspace"]

xx2 = x(5);  % One exact value
index = find(x == xx2);  % Of course this is 5
disp(y(index))

[lclk psdk]

i am so sorry for complexity the problem is that i plotted a figure of x , y then i want the program automatically uses the y value (any value) when its x value used (it is used in a loop) thanks alot for ur care

[lclk psdk]

thank u alot ,,, i found ur answer very useful,,,,,,, i only wanted to explain my question

[lclk psdk]

please why we wrote this,,,,,,,,,,,,,,,,, LineH = get(gca, 'Children'); x = get(LineH, 'XData'); y = get(LineH, 'YData');

[Image Analyst]

He's getting the data from the graph itself. It's as if you lost your original data, for example by calling the clear('x') command. If you did that unwise thing, then it's possible to extract your data from the graph, which has, in effect "stored" it.

[lclk psdk]

thank u very much............i am so happy to know such helpful men like u , Jan Simon and Ilham Hardy

Answer 3

[Ilham Hardy]

h = gcf; %handle of current fig
axObj = get(h, 'Children');
datObj = get(axObj, 'Children');
xdata = get(datObj, 'XData');
ydata = get(datObj, 'YData');

[lclk psdk]

thanks alot

Answer 4

[Asimananda Khandual]

Respected Image Analyst gave the best answer; I am just simplifying it with examples.

>> x=1:1:100; >> y=5*x+10; >> yDesired = interp1(x,y, 10)

yDesired =

    60

>> xDesired = interp1(y,x, 60)

xDesired =

    10

=====================================================================================

>> y=5*x.^2+2*x+10; >> plot(y,x) >> xDesired = interp1(y,x, 60)

xDesired =

    2.9630

>> yDesired = interp1(x,y, 10)

yDesired =

   530
-----------------HELPFILES---------------
Vq = interp1(X,V,Xq,METHOD) specifies alternate methods.
    The default is linear interpolation. Use an empty matrix [] to specify
    the default. Available methods are:

      'nearest'  - nearest neighbor interpolation
      'linear'   - linear interpolation
      'spline'   - piecewise cubic spline interpolation (SPLINE)
      'pchip'    - shape-preserving piecewise cubic interpolation
      'cubic'    - same as 'pchip'
      'v5cubic'  - the cubic interpolation from MATLAB 5, which does not
                   extrapolate and uses 'spline' if X is not equally
                   spaced.

    Vq = interp1(X,V,Xq,METHOD,'extrap') uses the interpolation algorithm
    specified by METHOD to perform extrapolation for elements of Xq outside
    the interval spanned by X.

    Vq = interp1(X,V,Xq,METHOD,EXTRAPVAL) replaces the values outside of the
    interval spanned by X with EXTRAPVAL.  NaN and 0 are often used for
    EXTRAPVAL.  The default extrapolation behavior with four input arguments
    is 'extrap' for 'spline' and 'pchip' and EXTRAPVAL = NaN (NaN +NaNi for
    complex values) for the other methods.

    PP = interp1(X,V,METHOD,'pp') will use the interpolation algorithm specified
    by METHOD to generate the ppform (piecewise polynomial form) of V. The
    method may be any of the above METHOD except for 'v5cubic'. PP may then
    be evaluated via PPVAL. PPVAL(PP,Xq) is the same as
    interp1(X,V,Xq,METHOD,'extrap').