e     F_a(x) = [ f(x,a+da) - f(x,a-da) ] / 2,

           with f(x) = the plotted curve
                'da' = error taken from the fit result
          Corr(a,a') = the correlation matrix from the fit result
                  Z = requested significance 'Z sigma band'

</pre>
The linear method is fast (requires 2*N evaluations of the curve,
where N is the number of parameters), but may not be accurate in the
presence of strong correlations (~>0.9) and at Z>2 due to linear and
Gaussian approximations made.
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