My client would like to be able to adjust all the values in a chart,
that is based on a large set of data, by some percentage depending on
various scenarios.

Is it possible to do this directly in the series formula? E.g.
something like...

=SERIES(Sheet1!$D$2,Sheet1!$E$1:$HG$1,Sheet1!$E$2: $HG$2*MULTIPLIER,1)

....where MULTIPLIER is a reference to a cell containing the percentage
variance.

Preempting the alternative suggestions of recalculating via VBA or
placing a formula in each datasource cell that calculates the product
of the multiplier and its original value - can anyone suggest which
method might be quicker on a range of 40,000 cells?

Br, Nick H