"Dynamic" Legends

I have a chart that has a series that simply illustrates the geometric mean
(as a flat line) spanning the seven days of a given month with the highest
7-day moving geometric mean. (So if the max 7-day geomean were on day 7, the
series has the value of that geomean showing from day 1 through day 7... when
a higher 7-day geomean pops up later, the line moves; so for example in
november the series displayed as a line segment with the value of the geomean
from days 19-25.)
Since this data is meaningless until we have 7-days of data, I would like to
have the line segment in the legend hidden; I've already taken care of the
legend wording to be a blank string if there's no data, but I still have a
fat brown line just sort of floating at the bottom of that chart.
All other series legends have been eliminated, this it the only one I want
to show and I only want it when it refers to valid data.
So I'd like to find a way to have the legend disapper early in the month. I
have a command button ("Set Up") that the users click to prepare the report
for a new shift's entries. I'm wondering if I can add some VBA to that.
Peace
EQC