androidgraphandroid-graphview

GraphView shows wrong graph in real time mode when scrollToEnd is true

I'm sampling accelerometer sensor and present a graph in real time, using Graph View library (4.0.1).

The app is working well except the graph is wrong:

enter image description here

As you can see in the picture, Z axis (magenta color) has value of ~9.8 but in the graph it's shows as ~15, Y axis (green color) has value of 0.2 but in the graph us under the zero and same idea with X axis.

Here is my code:

SensorManager sensorManager;
TextView tvX, tvY, tvZ;
GraphView graph;
LineGraphSeries<DataPoint> seriesX, seriesY, seriesZ;
long startTime;

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

    tvX = (TextView) findViewById(R.id.tvAcc_X);    // color : blue
    tvY = (TextView) findViewById(R.id.tvAcc_Y);    // color : green
    tvZ = (TextView) findViewById(R.id.tvAcc_Z);    // color : magenta
    graph = (GraphView) findViewById(R.id.graph);

    seriesX = new LineGraphSeries<>();
    seriesY = new LineGraphSeries<>();
    seriesZ = new LineGraphSeries<>();

    seriesX.setColor(Color.BLUE);
    seriesY.setColor(Color.GREEN);
    seriesZ.setColor(Color.MAGENTA);

    graph.addSeries(seriesX);
    graph.addSeries(seriesY);
    graph.addSeries(seriesZ);

    startTime = System.nanoTime() / 100000000;

    sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
    sensorManager.registerListener(this, sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER), SensorManager.SENSOR_DELAY_NORMAL);
}

@Override
public void onSensorChanged(SensorEvent event)
{
    tvX.setText(String.valueOf(event.values[0]));
    tvY.setText(String.valueOf(event.values[1]));
    tvZ.setText(String.valueOf(event.values[2]));

    updateGraph((event.timestamp / 100000000) - startTime,
            event.values[0], event.values[1], event.values[2]);
}

void updateGraph (final long timestamp, final float x, final float y, final float z)
{
    runOnUiThread(new Runnable() {
        @Override
        public void run() {
            seriesX.appendData(new DataPoint(timestamp, x), true, 40);
            seriesY.appendData(new DataPoint(timestamp, y), true, 40);
            seriesZ.appendData(new DataPoint(timestamp, z), true, 40);
        }
    });
}

I found the issue happened when scrollToEnd field is set to true, otherwise the graph is right. Another issue I found that may be related, the axis are not update when scro;lToEnd is true.

How can I beat the problem?

Solution

  • logcat shows why scrollToEnd does not work in your case:

    GraphView: scrollToEnd works only with manual x axis bounds

    to set the x axis bounds manual, add this code in your create method

    Viewport vp = graph.getViewport();
vp.setXAxisBoundsManual(true);
vp.setMinX(0);
vp.setMaxX(1000);

    but there is another issue with the code: System.nanoTime() and event.timestamp are different time formats

    System.nanoTime() // current value of system timer in nanoseconds
event.timestamp // nanoseconds since device uptime

    This is not very well documented: SensorEvent timestamp

    See also: Accelerometer SensorEvent timestamp