AndroidPlot background image/shader offset

Question

I've encountered something annoying with the background shader used as a background image: there is a width mismatch even after correcting the position offset.

A minimal project is available here: http://ge.tt/6UonbQ42/v/0

Here is a screenshot of the result:

The red line should be in sync with the background. It's OK for the first stops of the gradient but in the end, there is an offset. Is there a way to programatically know this offset or do I have to hardcode a guessed value which works for my device?

Here is the main code:

package com.example.androidplottest;

import java.util.ArrayList;

import android.app.Activity;
import android.graphics.Color;
import android.graphics.LinearGradient;
import android.graphics.Matrix;
import android.graphics.RectF;
import android.graphics.Shader;
import android.graphics.Shader.TileMode;
import android.os.Bundle;
import android.view.View;

import com.androidplot.xy.LineAndPointFormatter;
import com.androidplot.xy.SimpleXYSeries;
import com.androidplot.xy.XYPlot;
import com.androidplot.xy.XYSeries;

public class MainActivity extends Activity {

    public class Tuple<E> {
        public E x, y;
        public Tuple(E x, E y) {
            this.x = x;
            this.y = y;
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    public void drawPlot(View view) {
        XYPlot plot = (XYPlot) findViewById(R.id.plot);

        // Test data
        ArrayList<Tuple<Float>> data = new ArrayList<MainActivity.Tuple<Float>>();
        data.add(new Tuple<Float>(5f, 10f));
        data.add(new Tuple<Float>(5.1f, 150f));
        data.add(new Tuple<Float>(5.2f, 10f));
        data.add(new Tuple<Float>(5.3f, 150f));
        data.add(new Tuple<Float>(5.4f, 10f));
        data.add(new Tuple<Float>(9f, 150f));
        data.add(new Tuple<Float>(9.1f, 10f));
        data.add(new Tuple<Float>(9.2f, 150f));
        data.add(new Tuple<Float>(9.3f, 10f));
        data.add(new Tuple<Float>(9.4f, 150f));
        data.add(new Tuple<Float>(9.5f, 10f));
        data.add(new Tuple<Float>(9.6f, 150f));
        data.add(new Tuple<Float>(9.7f, 10f));
        data.add(new Tuple<Float>(9.8f, 150f));
        data.add(new Tuple<Float>(9.9f, 10f));
        data.add(new Tuple<Float>(10f, 150f));

        // Create series
        plot.addSeries(tupleArrayToXYSeries(data, "Data set"), new LineAndPointFormatter(Color.RED, null, null, null));

        // Find the dynamic range
        float max = data.get(0).y;
        float min = data.get(0).y;
        for(Tuple<Float> hb : data) {
            if(hb.y > max) max = hb.y;
            if(hb.y < min) min = hb.y;
        }       

        // Color bounds
        float minSat=0, maxSat=1;
        float minVal=0f, maxVal=1;

        // Target color
        int[] gradientColors = new int[data.size()];
        for(int i=0; i<data.size(); i++) {
            float hue = 120;
            float sat = Math.max(0, Math.min(1, (data.get(i).y-min)/(max-min)*(maxSat-minSat)+minSat));
            float val = Math.max(0, Math.min(1, (data.get(i).y-min)/(max-min)*(maxVal-minVal)+minVal));
            int color = Color.HSVToColor(new float[] {hue, sat, val});
            gradientColors[i] = color;
        }

        // Get times
        float[] timesArray = new float[data.size()];
        int i = 0;
        for (Tuple<Float> f : data) {
            timesArray[i++] = (f != null ? (f.x-data.get(0).x)/(data.get(data.size()-1).x-data.get(0).x) : Float.NaN);
        }

        // Set the plot background (shader translation due to this bug:
        // https://stackoverflow.com/questions/24738800/androidplot-background-image-shift)
        plot.getGraphWidget().setGridPadding(0, 0, 0, 0);
        plot.getGraphWidget().getGridRect();
        RectF rect = plot.getGraphWidget().getGridRect();
        Shader colorShader = new LinearGradient(0, 0, rect.width(), 0, gradientColors, timesArray, TileMode.CLAMP);
        Matrix m = new Matrix();
        m.setTranslate(rect.left, rect.top);
        colorShader.setLocalMatrix(m);
        plot.getGraphWidget().getGridBackgroundPaint().setColor(Color.WHITE);
        plot.getGraphWidget().getGridBackgroundPaint().setShader(colorShader);
        plot.redraw();
    }

    private XYSeries tupleArrayToXYSeries(ArrayList<Tuple<Float>> tuplesArray, String title) {
        ArrayList<Float> xList = new ArrayList<Float>();
        ArrayList<Float> yList = new ArrayList<Float>();
        for(Tuple<Float> tuple : tuplesArray) {
            xList.add(tuple.x);
            yList.add(tuple.y);
        }
        return new SimpleXYSeries(xList, yList, title);
    }
}