I have discovered the same problem using a combination of CategoryAxisRenderer and CanvasTickRenderer. I came up with the solution of formating the categories by hand in javascript and then passing them to the Renderer. In your case you could use the following as extender for your primefaces chart:
function extender() {
var millis = this.cfg.categories;
var dates = new Array(categories.length);
for (var i = 0; i < categories.length; i++) {
dates[i] = new Date(categories[i]).toLocaleDateString();
}
this.cfg.axes = {
xaxis : {
renderer : $.jqplot.CategoryAxisRenderer,
rendererOptions : {
tickRenderer : $.jqplot.CanvasAxisTickRenderer,
tickOptions : {
fontSize:'7pt'
},
ticks : dates
}
}
};
}