Thanks for the tip!

In the end I used timegm() to convert my datetime to epoch seconds, and just sucked these into GD::Graph::histogram, using x_number_format to deal with the x bin label formatting as a datetime not seconds. Note that this does not get executed unless you also set x_tick_number!

Results are fine; my only gripe is that the horizontal position of the label appears to drift to the right in the bin as the bin size count increases, so that I start with a centred bin label, and end up with the label hard on the right hand side.

Here's a code snippet:

my $graph = new GD::Graph::histogram(600,600);

$graph->set( 
    x_label         => 'Date and Time',
    y_label         => 'Messages',
    title           => 'Number of messages per bin',
    x_labels_vertical => 1,
    bar_spacing     => 0,
    shadow_depth    => 0,
    transparent     => 0,
    histogram_bins     => 10,
    show_values     => 1,
    x_number_format => \&x_format,
    x_tick_number => 10,
);

sub x_format {
    my $value = shift;
    # print $value . " " . localtime(int($value)) . " " . strftime( "%
+Y-%m-%d %H:%M:%S", localtime(int($value))) . "\n"; # test
    return strftime( "%Y-%m-%d %H:%M:%S", localtime(int($value)));
}

my $timevalues_ref = \@timevalues_secs;
my $IMG;
open($IMG, '> test_plot.png');
binmode $IMG;
print $IMG $graph->plot($timevalues_ref)->png;

close ($IMG);
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@timevalues_secs contain the timeseries data in epoch seconds.