Pseudodata for Vector Fields [Patch]

[Peter D. <pc...@wi...>]

Direction fields come in handy for first order ODEs; some people have
expressed frustration that they're not directly supported.*

The attached patch produces two dimensions of data from the pseudofile
operator when df_current_plot->plot_style == VECTOR, allowing you to
do things like:**

    set isosamples 20
    set samples 20
    l = 0.05
    f(t,y) = 2*y + 4 - t
    g(c,x) = -7/4. + 1/2.*x + c*exp(2*x)
    plot [0:2][-4:0] '+' using 1:2:(l):(l*f($1,$2)) \
         with vectors title "dy/dt - 2y = 4 - t", \
         g(0,x) title "g(c, t) = -7/4 + 1/2t + ce^2t, c = 0", \
         g(.1,x), g(.2,x), g(.3,x), g(-.1,x), g(-.2,x), g(-.3,x)

Here's the sourceforge tracker:

    http://sourceforge.net/tracker/index.php?func=detail&aid=1872528&group_id=2055&atid=302055

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* http://thread.gmane.org/gmane.comp.graphics.gnuplot.devel/5805/focus=5814
** http://wikisophia.org/wiki/User:Danenberg#Direction_field