Adds two tests for the add_xml_filters method of a plot object, a round
trip of a non-tree-modifying filter, and a simple tree modifcation that
adds a sub-plot of change bars to a bar plot.
The main plot title is given an additional class (plot_title) to
differentiate it from from the axis titles, so the it could for example
be styled in boldface using CSS.
Titles can be split at specified points with '\n' within the title
string. Titles segments which are still too long will still be split
over multiple lines in the normal way.
Allow formatting other than the default for the X axis on the DateY plots.
This is done by setting
datey.x_label_format = "%Y-%m-%d"
The formatting is done by the strftime function, so any valid format will work.
See: http://docs.python.org/2/library/time.html#time.strftime
An XML filter can be defined as a function taking a single argument,
supplied as the 'svg' atribute of the graph (an lxml etree?) and
returning a modified version/copy thereof.
XML filters can be added to a plot using the new plot method
add_xml_filter(func).
The hook that executes any filters added to the plot, executes them in
order immediately prior to conversion to a string, in the class method
svg.render, and also immediately prior to returning a copy of the tree
in BaseGraph.render_tree