How to find the point in a plot that indicate to the beginning of a curve?

Dear all,

I have a two dimension plot X, Y as shown in the attached image below. X and Y are linearly increase together, however at some point X will increase but y will not increase, and this simply will cause a curve in the plot.

My question is how to find the point X that indicate the beginning of the curve in the plot?

Any help will be appreciated.

Meshoo

Answer 1

[Mischa Kim]

Meshooo, you could use something like

 x  = 0:0.1:10;
 y  = sin(x);
 plot(x,y)
 xp = min(x(diff(y)<0))

diff computes the differences in array elements, sort of the first derivative of y. xp therefore marks the first x for which the derivative of y is negative.

[Meshooo]

Thank you very much Mischa. Would like to ask you what is the name of that point that I can get from your code? Is it the peak point?

Thank you.

Meshoo

[Mischa Kim]

From what I can see it should be the point that you marked in your plot. It is the x where y decreases for the "first" time.

[Meshooo]

Thank you very much Mischa.

I want to find the point where the plot takes a big change. In another words I want to find the point where the slope changes as I show here below.

Any idea will be appreciated.

[Meshooo]

XX = [143.3333
  144.3333
  145.3333
  146.9333
  147.1333
  148.2667
  149.1333
  150.0667
  151.3333
  152.4000
  153.6000
  154.9333
  156.2000
  157.5333
  159.0000
  160.4000
  161.8000
  163.2667
  164.6000
  165.9333
  167.4000
  168.8667
  170.3333
  171.7333
  173.2667
  174.8667
  176.4667
  177.9333
  179.4000
  180.9333
  182.1333
  183.3333
  184.6667
  186.0000
  187.1333
  188.6000
  189.8000
  191.2000
  192.4667
  194.0667
  195.3333
  197.2000
  198.7333
  200.4000
  201.9333
  203.7333
  205.2000
  206.8667
  208.6000
  210.2667
  211.7333
  213.3333
  215.0000
  216.6000
  218.0667
  219.8667
  221.6000
  223.1333
  224.7333
  226.4667
  227.9333
  229.3333
  230.8667
  233.2000
  234.3333
  236.3333
  237.9333
  239.2000
  239.6667
  241.0667
  241.6667
  242.8667
  244.0000
  245.0667
  246.8000
  248.4000
  249.9333
  251.5333
  253.1333
  254.1333
  254.1400
  254.1500
  254.1600
  254.1700
  254.1800
  254.1900
  254.2000
  254.2100
  254.2200
  254.2300
  254.2400
  254.2500
  254.2600
  254.2700
  254.2800
  254.2900
  254.3000
  254.3100
  254.3200
  254.3300
  254.3400
  254.3500
  254.3600
  254.3700]

YY = 1:104;

plot(YY,XX)

% if I apply your code

xp = min(YY(diff(XX)<0))

Answer is :

Empty matrix: 1-by-0