RT#127954: t/plot.t fails on Ubuntu 18.04

[mohawk2]

@eserte writes:

On my Ubuntu 18.04 smoker the test suite fails (with various perl versions):

...
# 
# P::G::G v2.01102, gnuplot v5.2, Perl v5.026001, /opt/perl-5.26.1/bin/perl on linux #
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 537.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 548.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 556.

******************************
Skipping 27 interactive tests.
    Set the environment variable GNUPLOT_INTERACTIVE to enable them.
******************************

#   Failed test 'Setting empty plot title sets an empty title'
#   at t/plot.t line 862.
Gnuplot warning: empty y range [0:0], adjusting to [-1:1]
 at t/plot.t line 969.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 1059.
INFO: Plotting to 'Plot-2.txt'
 at t/plot.t line 1062.
# Looks like you failed 1 test of 185.
t/plot.t ...... 
Failed 1/185 subtests 
        (less 32 skipped subtests: 152 okay)

and:

The test fails also on other systems. Currently it looks like the problem is gnuplot 5.2. Statistical analysis (in this case theta=0 means "bad"):

****************************************************************
Regression 'qr:gnuplot v([^,]+)'
****************************************************************
Name                   Theta          StdErr     T-stat
[0='const']           0.0000          0.0000       7.87
[1='eq_4.6']          1.0000          0.0000    20175114162040816.00
[2='eq_5.0']          1.0000          0.0000    20218736081426804.00
[3='eq_5.2']         -0.0000          0.0000      -5.29

R^2= 1.000, N= 68, K= 4
****************************************************************

[mohawk2]

This is probably as Slaven says, a 5.2 problem. Closing.