Gnuplot – Increase size of point only in legend/key

Here's a small example using inline data ...

plot '-' u 1:2:1 ps variable
  1 2
  2 3
  3 4
  4 5
  5 6
  e

Note that while it is possible to change the pointsize using a column from your datafile, I am not aware of any way to specify how large each point should be since the pointsize (and even the pointtype) is a terminal dependent quantity. The only workaround that I can come up with is to scale the points by a terminal dependent value.

default_value=1.0
func(x)=1.0*x  #Change this to change the functional dependence of the pointsize on x.
scale_point(x)=(GPVAL_TERM=="postscript") ? 1.0*func(x) :\
               (GPVAL_TERM=="png") ?        2.0*(x) :\
               default_value
plot "mydata.dat" u 1:2:(scale_point($1)) w pt ps variable

It's ugly, but it should get the job done -- You could also write a similar function which returns the pointtype to achieve some sort of terminal independence with that as well...

EDIT

Note, my points increase in size linearly as x gets bigger, but you can substitute any function you want to make the points get smaller as you see fit. What I am saying is that I don't believe that option exists since as you state "all this is terminal dependent". I tried to provide a ugly hack to allow you to modify this on a per-plot basis with minimal intervention -- allowing for different terminals, etc.

EDIT 2

While responding to your comment, I remembered an odd little corner of the gnuplot documentation that may be helpful.

set style fill transparent solid noborder  #whatever fillstyle you want...
plot 'mydata' u 1:2:3 w circles

see help circles ... I'm not sure when this feature was introduced, probably with gnuplot 4.3, but if you're using 4.2 then give it a try and let me know how it goes...

To adjust the length of the samples:

set key samplen X

(default is 4)

To adjust the vertical spacing of the samples:

set key spacing X

(default is 1.25)

and (for completeness), to adjust the fontsize:

set key font "<face>,<size>"

(default depends on the terminal)

And of course, all these can be combined into one line:

set key samplen 2 spacing .5 font ",8"

Note that you can also change the position of the key using set key at <position> or any one of the pre-defined positions (which I'll just defer to help key at this point)