Return png pattern: {BASE}.png {BASE}_pruned.png {BASE}_upper_pruned.png

def get_pltpat(self, plt_ext="svg"):
        """Return png pattern: {BASE}.png {BASE}_pruned.png {BASE}_upper_pruned.png"""
        if self.ntplt.desc == "":
            return ".".join(["{BASE}", plt_ext])
        return "".join(["{BASE}_", self.ntplt.desc, ".", plt_ext])