In
In_Use_Legend=hgdawe(bbT:ccT);
you didn't subscript your lookup array variables but refer to the arrays each in their entirety.
To make the above work would be
In_Use_Legend=hgdawe(bbT(i):ccT(i));
But, this is the wrong way to do something of the sort; as you're discovering char() arrays are arrays and need both indices and this is inconvenient at best.
Use cellstr() or the new(ish) string class here instead --
lgndName={'One','Two','Three','Four','Five','Six','Seven','Eight','Nine'}; % cellstr array
for i=1:numel(hgdawe)
plot(app.UIAxes2,In_Use_VertSec,In_Use_TVD,'DisplayName',lgndName(i));
end
