Hello,
> I want to draw a 3D graphic with the whole box around it (there are 3
> axes missing) and then I want to draw straight lines from the midpoint
> of the point to the axes and the grid respectively.
>
Some aspects of your plot requiere to write gnuplot code in the scene
description. If you want the complete box around the plot you need "set
border 4095". (Direct control from function draw could be available in
future releases.)
> So, I want to get a line that begins at 0.6 z-axis and ends at the
> midpoint of the first point. Then I want another line, that starts at
> 5 at the y-axis and 26 at the x-axis and ends in the midpoint of the
> first point.
>
If I understand correctly, try something like
draw3d(
/* the points */
color = grey,
point_size = 3,
point_type = 7,
points([[21, 0, .2]]),
color = red,
points([[18, -3, .2]]),
color = green,
points([[26,5,.6]]),
grid=true,
user_preamble="set border 4095",
/* the lines */
points_joined=true,
points([[26,5,.6],[0,0,0.6]]),
points([[26,5,.6],[0,5,0]]),
points([[26,5,.6],[26,0,0]]) )$
Note that point_size and point_type don't need to be rewritten, since
they will remain active until you change them again.
> Then I want another axis, beginning at (x=18,y=5), another beginning
> at (x=18,y=-3) and the last one beginning at (x=26,y=5).
>
> I think that the first point depends on the axis-options I don't find
> in the help. For the second I've no idea at all...
>
>
I'm not sure I understand this. Do you mean you want parallel lines to
the z-axis passing through points (18,5), etc of the xy-plane? In this
case, draw the extrems with points_joined set to true, as in the example
above.
The following options could be useful: xrange, yrange, zrange,
line_type, line_width, color, etc.
Let me know if you need some more help.
Viel Glueck!
--
Mario Rodriguez Riotorto
www.biomates.net