es on parameters from a fit
on any p.d.f. or function projection. To do so use the new <tt>VisualizeError()</tt> argument
in a plotOn call

<pre>
    RooFitResult* fr = pdf->fitTo(*data,Save(),...) ;
    pdf->plotOn(frame,VisualizeError(*fr),...) ;
</pre>

Two techniques for error visualization are implemented. The default is
linear error propagation, and results in an error band that is by
construction symmetric. The linear error is calculated as
<pre>

   error(x) = Z* F_a(x) * Corr(a,a') F_a'(x)

   wher