Counting unique rows based on dynamic 2-d array condition

[NJohn]

I have a massive dataset and am preparing a dashboard based on this dataset.
On my dashboard, I have a drop-down menu that allows me to select a month of my choice, from Jan to Apr. Here is a snippet of the data that I am looking to target my formula for.
<code style="margin: 0px; padding: 0px; border: 0px; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', 'Courier New', monospace, sans-serif; white-space: inherit;">Visitor Jan Feb Mar Apr
Jenny 2 3 0 1
Peter 2 0 1 3
Charley 0 2 4
Charley 1 2 2 3
Sam 1 4 2 3
Peter 2 2 5 0
John 3 3 6 9
Robin 4 0 7 0
</code>I am looking for a formula that will give me the number of unique visitors who have been active at least once in the month that I choose from the drop-down menu.
Hoping this is really clear, but if not, please feel free to shoot back your questions.
Thanks

 

[Domenic]

Assuming that A1:E9 contains the data, and that G2 contains the drop-down menu, try...

A1:H9

Visitor	Jan	Feb	Mar	Apr
Jenny	2	3	0	1		Mar	5
Peter	2	0	1	3
Charley	0	2	4
Charley	1	2	2	3
Sam	1	4	2	3
Peter	2	2	5	0
John	3	3	6	9
Robin	4	0	7	0

<tbody>
</tbody>

H2, confirmed with CONTROL+SHIFT+ENTER...

=SUM(IF(FREQUENCY(IF(INDEX($B$2:$E$9,0,MATCH(G2,$B$1:$E$1,0))>0,IF(LEN($A$2:$A$9)>0,MATCH("~"&$A$2:$A$9,$A$2:$A$9&"",0))),ROW($A$2:$A$9)-ROW($A$2)+1)>0,1))

Hope this helps!

 

[NJohn]

This is simply fantastic, Domenic. Just tried it and it is exactly what I was looking for. Helps a lot. Thanks a ton.

 

[Domenic]

You're very welcome!

Glad I could help!