Fix overlapping markers in feature plots

[teaguetomesh]

    def _close_line(self, line: matplotlib.lines.Line2D) -> None:
        x, y = line.get_data()
        # FIXME: markers at x[0], y[0] get doubled-up
        if x[0] != x[-1]:
            x = np.append(x, x[0])
            y = np.append(y, y[0])
            line.set_data(x, y)