There are several libraries for generating graphs with PHP, but I went with PHPlot. It is easy and highly customizable. Other graphing packages, such as JpGraph look like they would make more attractive graphs, but are either more complicated and/or not free for commercial use. I was only concerned with making bar graphs, but others are supported by PHPlot and most of the other packages.

Here is the end result:

And here is the PHP that generates it:

// This is to format the x values so that they decrement from left to right
function format_x_value($value, $numDays) {
	return $numDays - $value;
}

        // Every day at midnight, we insert a row in the table StatLog with the current
        // fighting stats, money, health, and level
	$query = "SELECT * FROM StatLog WHERE UserId='".$_SESSION["Id"]."' ORDER BY Date ASC";
	$result = mysql_query($query);

	if ($result) {
		$stat = "Level";
		if (isset($_GET["stat"])) {
			$stat = $_GET["stat"];
			if ($stat == "Potency" || $stat == "Girth" || $stat == "Resp" || $stat == "Level"
                            || $stat == "MoneyBank" || $stat == "TotalHealth" || $stat == "Total") {
				// no op
			} else {
				// not acceptable stat specified - use level by default
				$stat = "Level";
			}
		}

                // Use an array of arrays to make a bar graph with PHPlot
		$data = array();
		$index = 0;
		$numDays = mysql_num_rows($result);

		while ($row = mysql_fetch_assoc($result)) {
			if ($stat == "Total") {
				$data[$index] = array('', $index, $row["Potency"]+$row["Resp"]+$row["Girth"]);
			} else {
				$data[$index] = array('', $index, $row[$stat]);
			}
			$index++;
		}

		$plot = new PHPlot();
		// Only get one x tick per record
		$plot->SetNumXTicks($numDays-1);
		// Use formatting function to reverse number ordering
		$plot->SetXLabelType('custom', 'format_x_value', $numDays);
		// Set data to use for graph
		$plot->SetDataValues($data);
		// Set graph's title
		$plot->SetTitle($stat . " Log");
		// data-data is the type for a bar graph
		$plot->SetDataType('data-data');
		// Set x-axis title
		$plot->SetXTitle("Days Ago");
		// Set y-axis title
		$plot->SetYTitle($stat);
		$plot->SetDrawPlotAreaBackground("true");
		// Make bg transparent
		$plot->SetBackgroundColor('yellow');
		$plot->SetTransparentColor('yellow');
		// Set plot bg color
		$plot->SetPlotBgColor("DarkGreen");
		$plot->DrawGraph();
	} else {
		echo mysql_error();
	}

Then, from another page, the users can select the stat they would like a graph of and it is passed along like so:

<img src="viewgraph.php?stat=Potency" />

I’d be interested in hearing about other PHP graphing packages, but for now, I’m quite happy with these results.