Request that a key/value pair be considered for caching.

def _cache(self, key, val):
        """
        Request that a key/value pair be considered for caching.
        """
        cache_size = (1 if util.dimensionless_contents(self.streams, self.kdims)
                      else self.cache_size)
        if len(self) >= cache_size:
            first_key = next(k for k in self.data)
            self.data.pop(first_key)
        self[key] = val