Update StockUsageGraph.php

1. Set graph to back to bars. (original value, but was being assigned incorrectly)
2. Adjust left margin pixel value, suggested by @huguesderiau

StockUsageGraph.php

@@ -11,10 +11,10 @@
 $graph->SetYTitle(_('Quantity'));
 $graph->SetBackgroundColor("wheat");
 $graph->SetTitleColor("blue");
-$graph->SetPlotType('linepoints');
+$graph->SetPlotType('bars');
 $graph->SetShading(5);
 $graph->SetDrawYGrid(TRUE);
-$graph->SetMarginsPixels(40, 40, 40, 40);
+$graph->SetMarginsPixels(60, 40, 40, 40);
 $graph->SetDataType('text-data');
 
 if($_GET['StockLocation'] == 'All') {