Re: [Gnuplot-info] Finding thresholds with gnuplot

[Ethan A M. <eam...@gm...>]

On Thursday, 18 October 2018 20:45:18 theozh wrote:
> Hi,
> I would like to find the x-value of some data when its y-value passes a certain threshold.

Not clear.
Do you want one of these?

1) a list of all x values for which y(x) > threshold

2) an interpolated y value for each pair of x, x+1 where
   y(x) and y(x+1) are on opposite sides of the threshold

3) only the leading edge where y(x) < threshold and y(x+1) > threshold


> The curves can be very different, so I think that fitting to a certain function is probably not a solution.
> I know gnuplot is probably not made for that and other tools should be used. However, sometimes it is very convenient if there is a pure gnuplot solution.
> The basic idea is: 
> Subtracting the threshold from the data, take the square of this and look for the minimum.

I doubt that is what you really want.  It answers yet another question:  
  
4) for what x values is y(x) closer to threshold than both y(x-1) and y(x+1)?
Solutions to that do not necessarily involve crossing the threshold value at all.


> Maybe, I am trying way too complicated? If there are better ways please let me know.

It depends on what do you want to do with a value when you find it.
Do you need all points that satisfy your criterion or would any single
point suffice?

	Ethan

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