Showing ("00" .. "24") will be easy but I doubt it'll look very good. It also won't work as soon as you have more than 24 hours to display, or when you don't have enough space for all the numbers. I had a similar problem a few months back, here's how I solved it:

# format a set of dates for display on the X axis
sub dates_to_header {
    my ($self, $dates) = @_;
    my @header = ("") x scalar @$dates;

    # choose a format based on the length of the range
    my $delta  = $dates->[-1]->subtract_datetime($dates->[0]);
    my $months = $delta->delta_months;
    my $days = $delta->delta_days + ($months * 30);   # rough guess is
+ ok here

    my ($format, $number);
    if ($months >= 5 and $months <= 11) {
        $format = "%b";                               # month abrev
        $number = $months + 1;
    } elsif ($days <= 1) {
        $format = "%l %p";                            # HH AM/PM
        $number = 5;
    } elsif ($days <= 6) {
        $format = "%a";      # weekdays, mark them all
        $number = @$dates;
    } elsif ($dates->[0]->year eq $dates->[-1]->year) {
        $format = "%m/%d";    # MM/DD
        $number = 5;
    } else {
        $format = "%D";       # MM/DD/YYx
        $number = 5;
    }

    if ($number == @$dates) {

        # marking all bars
        return [map { $_->strftime($format) } @$dates];
    }

    # find the optimal spacing between labels
    my $spacing = @$dates / ("$number.0" - 1);

    # place a label at the edges
    my @spots = (0, $#$dates);

    # if there are odd numbers of dates to place, put one in the cente
+r
    my $center = int(@$dates / 2);
    push(@spots, $center) if $number % 2;

    # place the remaining dates around the center, following the spaci
+ng
    for (1 .. (($number - @spots) / 2)) {
        push(@spots, $center + int($spacing * $_));
        push(@spots, $center - int($spacing * $_));
    }

    # render the date at each chosen spot
    foreach my $spot (@spots) {
        $header[$spot] = $dates->[$spot]->strftime($format);
    }

    return \@header;
}
[download]

That subroutine takes a reference to an array of DateTime objects representing the data-points on the graph. It then applies a bunch of heuristics to come up with some reasonable labels for the X axis, leaving enough space for them to render successfully.

Hope that helps.

-sam