Achartengine for Android

I am using Achartengine to create a graph in my application. I want my X-axes to be string while I'm here:

public class LineGraph {
    public Intent getIntent(Context context){
        String[] x = { "a", "b" };
        int[] y = { 1 , 2 };

        TimeSeries series = new TimeSeries("Line");
        for(int i = 0; i < x.length; i++){
            series.add(x, y);
        }

        ...
    }
}