Try this. I will say, that you are going to run into a problem of having too many x values for the width of the graph, then you need to adjust this x_label_skip value to condense it. A Tk canvas would be a nice way to do this. You could plot every value on a scrolled canvas, and scroll to the current time(or time of interest). Of course if it's for the web, Tk won't help.

#!/usr/bin/perl
use warnings;
use GD::Graph::lines;
use strict;

my @time;
my @temp;
my @rad;

while(my $line = <DATA>){
 chomp $line;
 my ($celcius, $radiation, $time) = split(/ /, $line);
    push @temp, $celcius;
    push @rad, $radiation;
    push @time, $time;
 }

my $graph_data = [
           [@time], # X-values
           [@rad],  # Y-value 1
           [@temp], #Y-value 2 reference line
];

my $mygraph = GD::Graph::lines->new(600, 300);

$mygraph->set(
    transparent       => '0',
    bgclr             => 'white',
    boxclr            => 'lgray',
    fgclr             => 'white',
    x_label     => 'Time',
    y_label     => 'Value',
    title       => 'Temperature and Radiation',
    x_labels_vertical => 1,
#    x_label_skip => 1,
#    line_types  => [1, 1],
    line_width  => 2,
    dclrs       => ['blue', 'green'],
) or warn $mygraph->error;

$mygraph->set_legend_font(GD::gdLargeFont);

$mygraph->set_legend('Radiation','Temperature');

my $myimage = $mygraph->plot($graph_data) or die $mygraph->error;

open(GRAPH, ">$0.png");
print GRAPH $myimage->png;
close(GRAPH);
__END__
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