Creating dataset for Image_Graph

I have been pouring through the examples at pear.veggerby.dk and PHPBuilder.com and others but still don't quite understand how to get my dataset built for my graph. Can anyone help?

I want to create a multiple bar graph with y-axis in $ and x-axis in months. I want to have multiple bars at each month showing the production amounts for multiple years; similar to the example at pear.veggerby.dk for plot_bar_multiple .

My code so far looks like:

// create the graph
$Graph =& Image_Graph::factory('graph', array(600, 300));
// add a TrueType font
$Font =& $Graph->addNew('ttf_font', 'arial');
// set the font size to 11 pixels
$Font->setSize(8);

$Graph->setFont($Font);
$Graph->add(
Image_Graph::vertical(
Image_Graph::factory('title', array('Monthly Production', 12)),
Image_Graph::vertical(
$Plotarea = Image_Graph::factory('plotarea'),
$Legend = Image_Graph::factory('legend'),
90
),
5
)
);
$Legend->setPlotarea($Plotarea);
$histres = $db->query('SELECT
practice_id,
year,
month,
prod
FROM
month_history
WHERE
practice_id = '. $practice_id.'
ORDER BY
year, month');
if (PEAR::isError($histres)) {
$message .= "History: ".$histres->getMessage();
Var_Dump::display($message);
exit();
}
$doOnce = true;
$currentyear = 0;
$Dataset[] =& Image_Graph::factory('dataset');

while ($row = $histres->fetchRow()) {
if ($doOnce) {
$currentyear = $row['year'];
$doOnce = false;
}
if ($currentyear != $row['year']) {
$Dataset[] =& Image_Graph::factory('dataset');
$currentyear = $row['year'];
}
$Dataset->addPoint($month_abrev[$row['month']], $row['m_prod']);
}

// create the 1st plot as smoothed area chart using the 1st dataset
$Plot =& $Plotarea->addNew('bar', array($Dataset));

// output the Graph
$Graph->done();


When I run this, I get:
Fatal error: Call to a member function addPoint() on a non-object in /home/practicemonitor/test.php on line 126

line 126 is the addPoint call inside the while loop.

Any help out there? Please?

-Roxanne