Bluff SideBar Graph

Not sure if this issue is with how drupal uses the Bluff chart api or the Bluff chart api itself. When using the SideBar graph, the x axis values do not match to the data range in the graph, it always starts from 0 even though the smallest value in the graph is higher than this.
An example of this can be seen using the code below:

  $canvas = charts_graphs_get_graph('bluff');
  $canvas->title = "Bluff Chart";
  $canvas->type = "SideBar"; // a chart type supported by the charting engine. See further in the doc for the list.
  $canvas->y_legend = "Y Legend";
  $canvas->colour = '#D54C78';
  $canvas->theme = '37signals';  
  $canvas->series = array(
    'jan' => array(88),
    'feb' => array(86),
    'mar' => array(80),
    'apr' => array(82),
    'may' => array(88),
    'jun' => array(85),
    'jul' => array(89),
    'aug' => array(90),
    'sep' => array(92),
    'oct' => array(84),
    'nov' => array(83),
    'dec' => array(81),
  );
  $canvas->x_labels = array('humidity am');
  $out = $canvas->get_chart();
  return $out;

Using the code above creates a graph with x axis values from 0-90, but the bars are displayed from the minimum value in the data(80).
Therefore the bars do not match correctly with the x axis.

I hope this is explained clearly, but using the example code shows the problem clearly.