SELECT UNIX_TIMESTAMP(datetime_column) as unix_timestamp FROM table ORDER BY datetime_column DESC.

$query = "SELECT * FROM (SELECT * FROM outside ORDER BY id DESC LIMIT $limit) AS foo ORDER BY id ASC";

	    $result = mysql_query($query);
	    while($row = mysql_fetch_assoc($result))
	    {
	    $dataset1[] = array(($row['unixtime'] * 1000 ),$row['temperature']);
            }

• Good

1. I'm glad

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It's really simple. First you make a PHP thing to serve the data:

<?php
// this is get_data.php
$time = $_GET['time'];
$dataset1 = magic_function_to_fetch_outdoors_data_from_db($time);
$dataset2 = magic_function_to_fetch_inoors_data_from_db($time);

$data = array(
  "dataset1" => $dataset1,
  "dataset2" => $dataset2
);

echo json_encode($data);
?>

Then you modify your HTML links like so:

<ul id="period">
  <li><a href="get_data.php?time=1h">1 hour</a></li>
  <li><a href="get_data.php?time=2h">2 hour</a></li>
  <li><a href="get_data.php?time=6h">6 hour</a></li>
</ul>

Then you have some JS that looks something like this:

var $chart = $("#placeholder"),
    options = {
        lines: { show: true },
        points: { show: false }
        // etc.
    };

function plotChart(data) {
    $.plot($chart, [
        { data: data.dataset2, color: "#DE1B55", label: "Indoors" },
        { data: data.dataset1, color: "#5F2BCF", label: "Outdoors" }
    ], options);
}

$("#period").on("click", "a", function () {
    var $link = $(this);

    $link.addClass("working");

    $.ajax({
        url: this.href,
        type: "GET",
        dataType: "json"
    }).done(function (data) {
        plotChart(data);
    }).fail(function () {
        alert("O noes!");
    }).always(function () {
        $link.removeClass("working");
    });

    return false; // stops the browser from trying to follow the link
});

And that's basically it.