每次我调用这个方法时它都会返回nullpointerexception:org.achartengine.chart.XYChart.toScreenPoint(XYChart.java:867)出现java.lang.NullPointerException我看到图表的mScreenR为空如果不使用此方法toScreenPoint(double)，图表运行良好，这是代码:packagecom.insights.insights.gui;importjava.util.ArrayList;importorg.achartengine.ChartFactory;importorg.ach