This class constructs a histogram from specifications
in PlotML (Plot Markup Language), which is an XML language.
This class supports extends the base class to
ensure that the associated plotter is an instance of Histogram.
It ignores unrecognized elements in the DTD.
The class contains an instance of the Microstar Ælfred XML
parser and implements callback methods to interpret the parsed XML.
The way to use this class is to construct it with a reference to
a Plot object and then call its parse() method.
public void startElement(java.lang.String elementName)
Start an element.
This is called at the beginning of each XML
element. By the time it is called, all of the attributes
for the element will already have been reported using the
attribute() method. Unrecognized elements are ignored.