using the pseudo filename '+' twice gives strange results when the xrange is not set:
plot '+' using 0:1 w lp, '+' using 0:1 w lp
This error appears both in CVS and at least in 4.6.4.
I do not understand the intent of the plot command. Column 0 will run from 0 to \<current value of 'set sample'>, which defaults to 100 and gives the x axis range shown in your plot. But what is column 1? You haven't set an x range, so what exactly is supposed to be sampled, and what are you expecting to see as a y value in the plot?
I was expecting the + to sample the default xrange [-10:10], as it also does in the first plot command.
I had in mind, that a previous gnuplot version gave an error in this situation: using + without explicit xrange. But that is what happens with splot '++': -> "x range is invalid".
I think, if using plot '+' without explicitely setting an xrange is not supported, it should also give an error like for splot '++'.
I have not had occasion to use this mode myself, but I imagine the idea is something like this:
Consider the existing demo mgr . There is a file of observed values and a theoretical function that may or may not fit very well. The demo plots theory as a smooth line and data as points with experimental error bars. Suppose, however, that you had a denser data set and a theoretical but expersive to calculate error model. These commands would plot it:
set samples 11
set offset .1,.1
plot 'battery.dat' using 1:2 smooth csplines with lines,\
'+' using 1:(pred($1)):(error($1)) with yerrorbars
This is a very contrived plot, but the point is that '+' takes its sampling range from the autoscaled range of the previous plot element. That is exactly what your "strange result" does.
Thanks for the nice example.
At the moment I'm quite active at stackoverflow and there using '+' is very comfortable to demonstrate solutions related to data files.
I need to think a bit more about this:
Currently it seems to be as follows (also for the first plot):
I think the last point was also not clear to me. Don't know if there is something meaningful which one could do here, I'll see.