Display a heat calendar for a month of data.

monthmap.php

<?php

// Create an array of timestamps for posts
// and put them into an array called $timestamps.

foreach ($timestamps as $timestamp) {
    $day = date("j", $timestamp);
    $hour = date("G", $timestamp);
    if (isset($heatcalendar['posts'][$day][$hour])) {
        $heatcalendar['posts'][$day][$hour]++;
    } else {
        $heatcalendar['posts'][$day][$hour] = 1;
    }
}

$heatcalendar['days'] = date("t", $timestamps[0]);
$heatcalendar['month'] = date("F", $timestamps[0]);
$heatcalendar['hours'] = 4; // How many hours in a "slice" of a 24 hour day

echo "<div style=\"max-width: 100%; overflow: auto\">\n";
echo "<table class=\"meta\" style=\"width: 100%;\" summary=\"Posts by time of day\">\n";
echo "<thead>\n";
echo "<tr>\n";
echo "<td></td>\n";
for ($day = 1; $day <= $heatcalendar['days']; $day = $day + 1) {
    echo "<th scope=\"col\" id=\"day".$day."\" aria-label=\"".$heatcalendar['month']." ".$day."\">";
    if ($day % 5 == 0) {
        echo $day."th";
    } else {
        echo "&nbsp;&nbsp;&nbsp;&nbsp;";
    }
    echo "</th>\n";
}
echo "</tr>\n";
echo "</thead>\n";
echo "<tbody>\n";
for ($hour = 0; $hour < 24; $hour = $hour + $heatcalendar['hours']) {
    echo "<tr>\n";
    echo "<th scope=\"row\" id=\"hour".$hour."\">";
    echo date("ga", strtotime($hour.":00"));
    echo "</th>\n";
    for ($day = 1; $day <= $heatcalendar['days']; $day = $day + 1) {
        $total = 0;
        for ($i = 0; $i < $heatcalendar['hours']; $i = $i + 1) {
            if (isset($heatcalendar['posts'][$day][$hour + $i])) {
                $total = $total + $heatcalendar['posts'][$day][$hour + $i];
            }
        }
        if ($total > 0) {
            echo "<td title=\"";
            echo $total;
            echo " post";
            if ($total != 1) {
                echo "s";
            }
            echo "\" headers=\"day".$day." hour".$hour."\">";
            echo "<span style=\"all: initial; border-radius: 50%; width: 1em; height: 1em; background-color: currentcolor; display: inline-block; transform: scale(";
            echo 0.5 + (0.1 * $total);
            echo "); opacity: ";
            echo 0.1 + (0.1 * $total);
            echo "\" aria-describedby=\"day".$day." hour".$hour."\" aria-label=\"";
            echo $total;
            echo " post";
            if ($total != 1) {
                echo "s";
            }
            echo "\">";
            echo "&nbsp;";
            echo "</span>";
        } else {
            echo "<td>";
        }
        echo "</td>\n";
    }
    echo "</tr>\n";
}
echo "</tbody>\n";
echo "</table>\n";
echo "</div>\n";
echo "<br />\n";

?>