I googled, thinking "cusum" might be a typo for "custom", but it turns
out that "cusum" is a cumulative sum graph, a sort of alternative type
of control graph, and the V mask is the equivalent of the upper and
lower control lines. But, because this is a cumulative graph, instead of
being horizontal, they get closer together from left to right.

It wasn't clear to me, from the pictures I saw, whether the V lines are
straight or curved. If straight, this is just another case where a pair
of points, in a block of cells kept apart from the data columns, define
a line in an XY scatter graph. If curved, then they'll probably have to
be a series, and might as well be in spreadsheet columns alongside the
data.

Hi,

Well for the control chart there are explanations here,
http://processtrends.com/pg_charts_control_chart.htm
http://peltiertech.com/Excel/Charts/RunChtLines.html

As for a particular formula to calculate the v-mask lines we would need
much more detail.

Cheers
Andy