[Fwd: Re: Difference between 2 lines and adaptative grid]

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[Fwd: Re: Difference between 2 lines and adaptative grid]

Grégory Lielens
On Mon, 2005-08-29 at 17:39 +0200, Nicolas Girard wrote:

> Hi all,
> I apologize for beeing slightly off-topic with this question, but I couldn't
> think about a better place to ask :-/
> Consider two vectors x1 and y1. Using matplotlib, plot(x1,y1) will display the
> line joining all points whose coordinates are (x1(i),y1(i))
> Now, consider another couple of vectors x2 and y2.
> Using plot(x1,y1,x2,y2) we are shown the 2 lines corresponding to (x1,y1) and
> (x2,y2), and we can compare these two lines with the naked eye
> My question is, how to do this quantitatively ? Is there a way of calculating
> the difference between the two sets of points, and then plot this difference,
> using matplotlib ? I mean, if the data was generated using a fixed grid, it
> would be enough to plot (y2-y1) but with an adaptative grid, x1 and x2
> differ. Do you know about a quick & dirty way to achieve this ?

You have to interpolate the results to a common vector, and then perform
the difference. If you want to reproduce exactly the difference you can
see on-screen, then use a linear interpolation (cause the lines joining
successive points are straight, hence a linear interpolation -  well,
assuming that your plot is in cartesian coordinates, else this becomes
more tricky), and map both set of results to the common vector
(list(x1)+list(x2)).sort() (remove duplicated values and reduce it to
the interval max(min(x1),min(x2)), min(max(x1),max(x2)) if you want to
be clean and safe...)

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