I seem to remember the following variations in \startuseMPgraphic.
(1) \startuseMPgraphic{name}{comma separated list of variables}
(2) \startuseMPgraphic{name}{comma separated list of key=value pairs}
In the second case one is able to set a default value to that variable, possibly to be overridden with \useMPgraphic{name}{key=value}.
I very much would like to specify default values on the \startuseMPgraphic definition of the graphic.
However, I cannot get option (2) working. Do I have a bad memory (possible of course ;-), Has it to be done otherwise?