Two and a half years after someone reported this bug I also run into it. Here is the relevant part of the backtrace with JRobin 1.5.9.1:
java.lang.ArrayIndexOutOfBoundsException: -1
at org.jrobin.graph.ValueAxis.draw(ValueAxis.java:129)
at org.jrobin.graph.RrdGraph.drawGrid(RrdGraph.java:212)
at org.jrobin.graph.RrdGraph.createGraph(RrdGraph.java:93)
at...