How to create a cumulative posts and members count

Here is a very crude script I’ve knocked up to get what you are after:

<?php
require('wp-blog-header.php');

$posts = get_posts('numberposts=-1&order=ASC');

$posts_times = array();

foreach ($posts as $post) {
    $post_time = strtotime($post->post_date);
    $offset = $post_time % (60*60*24);
    $post_time -= $offset;
    $posts_times[$post_time]++;
}

$keys = array_keys($posts_times);

$running_count = 0;

$end_data = array();

for($i = $keys[0]; $i <= $keys[(count($keys)-1)]; $i += (60*60*24)) {
    $running_count += $posts_times[$i];
    $end_data[] = array(
        array(date("Y", $i), date("m", $i), date("d", $i)),
        $running_count
    );
}
echo "<pre>";
print_r($end_data);
?>