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Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: units when plotting ellipses
Date: Fri, 1 Mar 2019 21:23:26 +0100
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On 28.02.19 17:51, Nicholas Patterson wrote:
> I am trying to use 
> plot datafile with ellipses 
> 
> major and minor axes are in x-units or y-units (depending on 
> set style ellipses settings).  But I don't know what the units are (or even mean)
> and the coordinates of my plotted ellipse depend (for instance) on xrange and 
> yrange.  I know the absolute coordinates and that's what I want.  
> Help please!   For instance I want to plot an ellipse, center at origin
> major axis 10, minor 2 and major aligned with y-axis.  
> datafile is:
> 0 0 10 2 90   
> But this doesn't work as I alter yrange. 

If you want that both axis aligned with the y-axis try

plot datafile with ellipses units yy

Be careful with the rotation. First the ellipse will be draw and than it
will be rotated, so x-y allocation is changed.

Example with your datafile:

plot 'ellipse.dat' u 1:2:4:3:5 w ellipses units yy

gives a ellipse with the following dimensions to you:

center 0,0
focus is at -2.5 and 2.5
distance between focus 1 and 2 is 10 (This is not the distance on the
major axis, see https://en.wikipedia.org/wiki/Ellipse)
minor axis is 2


Jörg