Build the props dictionary.

def build_props(self):
        """Build the props dictionary."""
        props = {}
        if self.filters:
            props["filters"] = {}
            for grp in self.filters:
                props["filters"][grp] = [f.params for f in self.filters[grp]]
        if self.charts:
            props["charts"] = [c.params for c in self.charts]

        props["type"] = self.layout
        return props