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From: roby on 21 Oct 2009 06:38
Hi Janey

in yor case this may help.

field=VectorPlot[{f[x,y],g[x,y]},{x,0,70},{y,0,2400},VectorPoints->
{15,15},Frame->True,AspectRatio->1,VectorScale->{.001,2,Automatic}]



The behaviour of the new VectorPlot is not very enjoining with respect
to compatibility to the old VectorFieldPlot.

What is missing in VectorPlot is a special Value for the unitlen in
the VectorScale Option, it should be possible to provide None to
indicate that no scaling has to be performed at least no scaling due
to unitlen.

the Option: VectorScale->{unitlen,aratio,sfun}
would need: VectorScale->{None, Automatic, Automatic} but this sadly
is not supported.

Any WRI-Guru listening ?

Regards Robert


On 19 Okt., 13:08, janey <janemk...(a)gmail.com> wrote:
> Dear Mathematica Usenet Group,
>
> I have been having a lot of problems with finding appropriate options
> in Version 7 for VectorPlot that give an acceptable, viewable plot
> when scales on the horizontal and vertical axes are different (I am
> trying to get predator prey phase portraits, plotting population of
> predator vs population of prey; obviously there are far more prey than
> predators).
>
> My only solution so far has been to put up with the complaints from
> using
> << VectorFieldPlots`
> about the version 6 legacy version.
>
> For example, the following works just fine (apart from annoying
> warnings) in version 7 given that it uses the version 6 command
> VectorFieldPlot in place of version 7's VectorPlot:
>
> << VectorFieldPlots`;
>
> f[x_, y_] = -.05 x + .0001 x y; g[x_, y_] = .1 y - .005 x y;
>
> field = VectorFieldPlot[{f[x, y], g[x, y]}, {x, 1, 70}, {y, 0, 2400},
> PlotPoints -> {15, 15}, Frame -> True, AspectRatio -> 1]
>
> With Version 7's VectorPlot, although I can specify the number of
> arrows to be shown with VectorPoints replacing PlotPoints, I have not
> been able to set VectorScale to give me anywhere near an acceptable
> plot.
>
> So in version 7,
>
> f[x_, y_] = -.05 x + .0001 x y; g[x_, y_] = .1 y - .005 x y;
>
> field = VectorPlot[{f[x, y], g[x, y]}, {x, 1, 70}, {y, 0, 2400},
> VectorPoints -> {15, 15}, Frame -> True, AspectRatio -> 1]
>
> looks very bad.
>
> Is there some source of information for manipulating these options
> somewhere besides the Mathematica Documentation Center's minimalist
> examples? If not, is anyone willing to share how to get success with
> this?
>
> Any help you can give me would be much appreciated,
> thanks
> Janey.


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