Im using all the code verbatim from the tutorial, but I am getting these errors when the page loads:

Caused by: java.lang.IllegalArgumentException: only absolute URLs or full path URIs are allowed
    at org.apache.pluto.core.impl.PortletResponseImpl.encodeURL(PortletResponseImpl.java(Compiled Code))
    at javax.servlet.http.HttpServletResponseWrapper.encodeURL(HttpServletResponseWrapper.java(Compiled Code))
    at de.laures.cewolf.taglib.tags.ChartImgTag.buildImgURL(ChartImgTag.java:173)
    at de.laures.cewolf.taglib.tags.ChartImgTag.getImgURL(ChartImgTag.java:200)

Hopefully someone can help!