I don't know much about Gnuplot and I've a problem with this super
simple command:

plot "./spa.txt" u 4:2 w l

Col 2 & 4 contain sequential series with the same starting and ending
T points. Here are 3 of them:

SP= 57.050000      T= 0
.....
SP= 36.232421      T= 0.92
SP= 57.050000      T= 0
.....
SP= 70.035272      T= 0.92
SP= 57.050000      T= 0
.....
SP= 88.291745      T= 0.92

The series are correctly plotted but each end point is re-connected to
the origin with a straight line.

How can avoid plotting those unwanted straight lines?

MamboKnave wrote:
> Col 2 & 4 contain sequential series with the same starting and ending
> T points. Here are 3 of them:
> 
> SP= 57.050000      T= 0
> ....
> SP= 36.232421      T= 0.92
> SP= 57.050000      T= 0
> ....
> SP= 70.035272      T= 0.92
> SP= 57.050000      T= 0
> ....
> SP= 88.291745      T= 0.92

The problem is that this bears no indication of being three of anything. 
It's a single sequence of points with no breaks or anything in it. 
gnuplot has no way of knowing that (57.05, 0) is some special point for you.

> How can avoid plotting those unwanted straight lines?

By telling gnuplot about the intended start of a new polyline.  The way 
to do that is explained in "help data".