check for NPE

xchart/src/main/java/com/xeiam/xchart/internal/chartpart/Axis.java

@@ -279,7 +279,7 @@ public class Axis implements ChartPart {
       String sampleLabel = axisTickCalculator.getTickLabels().get(0);
       // find the longest String in all the labels
       for (int i = 1; i < axisTickCalculator.getTickLabels().size(); i++) {
-        if (axisTickCalculator.getTickLabels().get(i).length() > sampleLabel.length()) {
+        if (axisTickCalculator.getTickLabels().get(i) != null && axisTickCalculator.getTickLabels().get(i).length() > sampleLabel.length()) {
           sampleLabel = axisTickCalculator.getTickLabels().get(i);
         }
       }