无法从我的数据列表中绘制我的LineChart

Question

下面是显示我的LineChart的代码 从Firebase检索一组数据后，用于绘制折线图。不知怎的，我无法将我的数据集显示到x和y轴以及LineChart本身。请帮忙。我的x轴是“Date：mm / dd / yy”，而我的y轴是Float Data

private void displayGraph() {

        //Storage for (String) dates from Firebase: public ArrayList<String> x_axis = new ArrayList<>();
        //public ArrayList<String> y_axis = new ArrayList<>()
        ArrayList<String> xVals = new ArrayList<>();
        ArrayList<Entry> yVals = new ArrayList<>();
        int times = 0;

        //date as for X - axis
        for (String xaxis : x_axis) {
            xVals.add(xaxis);
        }

        //values from my y_axis ArrayList to Y - axis
        for (String item : y_axis) {
            yVals.add(new Entry(Float.valueOf(item), times));
            times++;
        }

        //for Y axis
        LineDataSet yValue = new LineDataSet(yVals, "Consumed");

        LineData xydata = new LineData(xVals, yValue);
        linechart.setData(xydata);
        linechart.getXAxis().setTextColor(Color.WHITE);
        linechart.invalidate();

    }