Greetings,

I've created a utility using Statistics::LineFit and another using Gnuplot and fed both the same sample data. The results differ, so I must have made a mistake, but I can't see where.

Perl code

#!/usr/bin/perl

use strict;
use warnings;
use Statistics::LineFit;
use Time::Local;
use Data::Dumper;

my @x_axis;
my @y_axes;

sub date_to_epoch
{
   my $date = shift;
   my ( $y, $m, $d ) = split /-/, $date;
   return timelocal( '59', '59', '23', $d, $m, $y );
   #return timelocal( '0', '0', '0', $d, $m, $y );
}
  
sub max_value
{
   my @array = @_;
   my $max = $array[0];

   for ( my $i = 0; $i <= $#array; $i++ )
   {
      $max = $array[$i] if ( $array[$i] > $max );
   }
   return $max;
}

my @epochs;

while (<DATA>)
{
   next if ( m/^#/ );
   chomp;

   if ( my @line = split /\s+/ )
   {
      my $epoch = date_to_epoch( $line[0] );
      
      # factor down epoch or slope is too shallow.
      push @x_axis, $epoch;
      shift @line;

      for ( my $y = 0; $y <= $#line; $y++ )
      {
          push @{$y_axes[$y]}, $line[$y] ;
      }
   }
}

print Dumper ( \@x_axis );
print Dumper ( \@y_axes );

my $lineFit = Statistics::LineFit->new( 0, 0 ); # TODO change 2nd to 1
$lineFit->setData( \@x_axis, \@{$y_axes[0]} ) or die "Invalid regressi
+on data\n";
my ( $intercept, $slope ) = $lineFit->coefficients();
print "Slope(m): $slope  Y-intercept(b): $intercept\n";

my %fitline;
$fitline{y1} = $intercept;
$fitline{x1} = 0;
$fitline{y2} = max_value( @{$y_axes[0]} );
$fitline{x2} = ( $fitline{y2} - $fitline{y1} ) / $slope + $fitline{x1}
+;

print Dumper ( \%fitline  );

__DATA__
# date     notkept hosts
2014-04-01 50      10
2014-04-02 63      11
2014-04-03 120     12
2014-04-04 55      20
2014-04-05 60      22
2014-04-06 63      25
2014-04-07 52      24
[download]

Gnuplot

#!/usr/bin/gnuplot

#set output "test.png"
set title "Promises not kept"
set xlabel "Date"
set ylabel "Count"
set rmargin 7

set border linewidth 2
set style line 1 linecolor rgb 'blue' linetype 1 linewidth 2
set style line 2 linecolor rgb 'black' linetype 1 linewidth 2
set style fill solid

set xdata time
set timefmt "%Y-%m-%d"
set format x "%Y-%m-%d"
set grid front
set grid
set autoscale

# 1e8 reduces the epoch seconds for a less flat line.
h(x) = m2 * x + b2
fit h(x) 'test.dat' using 1:3 via m2,b2
p(x) = m1 * x + b1
fit p(x) 'test.dat' using 1:2 via m1,b1

#set terminal png enhanced size 1024,768
plot 'test.dat' using 1:2 title 'Promises not kept' with boxes lc rgb 
+"orange", \
p(x) title 'Promise Trend' with lines linestyle 1, \
h(x) title 'Host Trend' with lines linestyle 2
[download]

test.dat

# date     notkept hosts
2014-04-01 50      10
2014-04-02 63      11
2014-04-03 120     12
2014-04-04 55      20
2014-04-05 60      22
2014-04-06 63      25
2014-04-07 52      24
[download]

Perl results

$VAR1 = [
          1399003199,
          1399089599,
          1399175999,
          1399262399,
          1399348799,
          1399435199,
          1399521599
        ];
$VAR1 = [
          [
            '50',
            '63',
            '120',
            '55',
            '60',
            '63',
            '52'
          ],
          [
            '10',
            '11',
            '12',
            '20',
            '22',
            '25',
            '24'
          ]
        ];
Slope(m): -2.23214285714286e-05  Y-intercept(b): 31299.6785491071
$VAR1 = {
          'y1' => '31299.6785491071',
          'x2' => 1396849599,
          'y2' => 120,
          'x1' => 0
        };
[download]

Gnuplot results

Final set of parameters            Asymptotic Standard Error
=======================            ==========================

m1              = 1.44796e-07      +/- 5.823e-05    (4.022e+04%)
b1              = 1                +/- 2.62e+04     (2.62e+06%)

correlation matrix of the fit parameters:
               m1     b1
m1              1.000
b1             -1.000  1.000
[download]

Note that m1 and b1 from gnuplot are not the same as Slope and Y-intercept from Perl. Why?