How to get contour or surface plot form datafile (with gnuplot)?

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How to get contour or surface plot form datafile (with gnuplot)?

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I would like to use gnuplot to plot my 2d function ch(x,y). My datafile is structured in this way:

x1 y1 ch(x1,y1)
x2 x2 ch(x2,y2)
x3 y3 ch(x3,y3)
x1 y1 ch(x1,y1)

x4 y4 ch(x4,y4)


where I have the value of my function at the 3 vertices of every triangle which constitutes my (triangular, not structured) mesh (see figure here http://i65.tinypic.com/2mydkq9.jpg).
What I would like to get is two separate figures:
1. something like this for the surface, with a colorbar as a legend:
2. something like this for contour lines, with a colorbar as a legend:

How can I get these two figures with gnuplot? 
I've tried for example doing the first one :

    set palette rgbformulae 33,13,10
    set xrange [0: 0.25]
    set yrange [0: 0.20]
    set view map
    splot "mydatafile.txt" w l pal 

but the triangles inside are white (not filled with colors) and only the edges of the triangles are coloured.
How about the contour lines?

Thank you in advance,