As the labels remain the same for the two datasets, just add the second (and subsequent) set(s) of values to the first array:

#! perl -slw
use strict;
use GD::Graph::bars;

my @d1 = (
    [ "2010/03/29 14:00:00", "2010/03/29 14:00:00" ],
    [ 13, 15 ],
    [ 20, 21 ],
    [ 4, 9 ],
);

my $graph = GD::Graph::bars->new(1000, 800);

open PNG, '>:raw', "$0.png" or die $!;
print PNG $graph->plot( \@d1 )->png;
close PNG;
system "$0.png";

[download]

Produces:this graph.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

The start of some sanity?

In reply to Re: How to add two array data into one GD plot? by BrowserUk
in thread How to add two array data into one GD plot? by heinz_zh

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.