Different behaviors in overlay vs underlay when it comes to updating. Create a 1D selection by right-dragging on the plot, and try dragging the selection back and forth. This works as expected when the selection is drawn using an overlay, but not when it is drawn as an underlay.

over_underlay.py

import numpy as np

from chaco.api import ArrayPlotData, Plot, VPlotContainer
from chaco.tools.api import RangeSelection, RangeSelectionOverlay
from enable.api import Component, ComponentEditor
from traits.api import Enum, HasTraits, Instance
from traitsui.api import UItem, View


class A(HasTraits):

    plot = Instance(Component)
    which = Enum(["overlays", "underlays"])
    view = View(
        UItem("which"),
        UItem("plot", editor=ComponentEditor()),
    )

    def _create_plots(self):
        container = VPlotContainer(use_backbuffer=True)
        plot_data = ArrayPlotData(
            index=np.arange(1000), data=np.arange(1000),
        )
        plot = Plot(plot_data)
        renderer = plot.plot(("index", "data"))[0]

        getattr(renderer, self.which).append(
            RangeSelectionOverlay(component=renderer)
        )
        tool = RangeSelection(component=renderer)
        renderer.tools.append(tool)

        container.add(plot)
        return container

    def _which_changed(self):
        self.plot = self._create_plots()

    def _plot_default(self):
        return self._create_plots()


if __name__ == '__main__':
    A().configure_traits()