You can pass whatever you want to the parser; it doesn't get automatically what is passed to the function, but works on what you pass to its parse method. You have therefore the freedom to do something like:
function foo( varargin )
% - Extract first optional axes handle. Remove if present.
if ~isempty( varargin ) && ishandle( varargin{1} )
ax = varargin{1} ;
varargin = varargin(2:end) ;
else
ax = [] ;
end
% - Build input parser and parse remaining args.
parser = inputParser ;
parser.StructExpand = true ;
parser.CaseSensitive = false ;
parser.addRequired( .. ) ;
.. etc.
parser.parse( varargin{:} ) ; % Pass only remaining args.
args = parser.Results ;
% - ...
end
