Contour plot does not work with logarithmic scale

The `contour_plot` method is not correctly scaled in a logarithmic plot. Here is a modified example that exhibits this bug:

``` python
#!/usr/bin/env python
"""
This example illustrates a problem with the contour_plot with a logarithmic
axis scaling. This file is a modification of contour_plot.py

Draws a contour polygon plot with a contour line plot on top
- Left-drag pans the plot.
- Mousewheel up and down zooms the plot in and out.
- Pressing "z" brings up the Zoom Box, and you can click-drag a rectangular
region to zoom. If you use a sequence of zoom boxes, pressing alt-left-arrow
and alt-right-arrow moves you forwards and backwards through the "zoom
history".
"""

# Major library imports
from numpy import cosh, exp, linspace, meshgrid, pi, tanh, sin

# Enthought library imports
from enable.api import Component, ComponentEditor
from traits.api import HasTraits, Instance
from traitsui.api import Item, Group, View

# Chaco imports
from chaco.api import ArrayPlotData, jet, Plot
from chaco.tools.api import PanTool, ZoomTool

#===============================================================================
# # Create the Chaco plot.
#===============================================================================
def _create_plot_component():
# Create a scalar field to contour
xs = linspace(1, 100, 600)
ys = linspace(0, 10, 300)
x, y = meshgrid(xs,ys)
z = x*y

# Create a plot data obect and give it this data
pd = ArrayPlotData()
pd.set_data("imagedata", z)

plot = Plot(pd, default_origin="top left")

# Add a line plot with a log scale
pd.set_data("xline", xs)
pd.set_data("yline", ys*100)
plot.plot(("xline", "yline"), color="blue", index_scale="log")

# Create a contour line plot for the data, too
cplot = plot.contour_plot("imagedata", type="line")

# Tweak some of the plot properties
plot.title = "Drag and drop the plot to see how the black contour plot gets rescaled (bug).\n It should behave like the blue line (reference)."
plot.bg_color = "white"
plot.fill_padding = True

# Attach some tools to the plot
plot.tools.append(PanTool(plot))
zoom = ZoomTool(component=plot, tool_mode="box", always_on=False)
plot.overlays.append(zoom)
return plot

#===============================================================================
# Attributes to use for the plot view.
size = (800, 600)
title = "Contour plot logarithmic scaling bug"

#===============================================================================
# # Demo class that is used by the demo.py application.
#===============================================================================
class Demo(HasTraits):
plot = Instance(Component)

traits_view = View(
Group(
Item('plot', editor=ComponentEditor(size=size),
show_label=False),
orientation = "vertical"),
resizable=True, title=title
)

def _plot_default(self):
return _create_plot_component()

demo = Demo()

if __name__ == "__main__":
demo.configure_traits()

#--EOF---
```


I have also tried the `index_scale="log"` keyword argument, but it does not change anything.