Vertical Grid lines not showing

[FlashBulb]

I am creating charts with Actionscript, and I have this code to create grid lines:
var chartGrid:GridLines = new GridLines();
var xgridStroke:Stroke = new Stroke(vars.x_grid_colour, 3);
var ygridStroke:Stroke = new Stroke(vars.y_grid_colour, 3);
chartGrid.setStyle("horizontalStroke", xgridStroke);
chartGrid.setStyle("verticalStroke", ygridStroke);

myChart.backgroundElements = [chartGrid];

The horizontal grid shows up, but I can't get any vertical grid lines to show. what am I doing wrong??