Two Y axes (same scale) in ggplot2 or plot

Dear R users,

I'm plotting housing prices in City A over past 30 years in ggplot2. The Xs
are years since 1980. I have two housing price variables: new home prices
and old home prices, both of them measured by $/sqft. I have searched
related threads on multiple Y axes in ggplot2 and I understand that
multiple Y axes in different scales are not possible. I'm wondering if it
is possible to have multiple Y axes with the same scale in ggplot2, like in
my case. If still not possible, is there a easy way to do it in R's default
plot function? Thanks.

Thanks, Duncan.

I tried axis(). It appears it allows you to add an axis, but does not 
say you can plot a second Y in the graph. Maybe I'm understanding it 
correctly. Any help will be appreciated!

Gary


On Wed, May 9, 2012 at 7:26 AM, Duncan Murdoch 
<murdoch.duncan at gmail.com <mailto:murdoch.duncan at gmail.com>> wrote:

    On 08/05/2012 3:23 PM, Gary Dong wrote:

        Dear R users,

        I'm plotting housing prices in City A over past 30 years in
        ggplot2. The Xs
        are years since 1980. I have two housing price variables: new
        home prices
        and old home prices, both of them measured by $/sqft. I have
        searched
        related threads on multiple Y axes in ggplot2 and I understand
        that
        multiple Y axes in different scales are not possible. I'm
        wondering if it
        is possible to have multiple Y axes with the same scale in
        ggplot2, like in
        my case. If still not possible, is there a easy way to do it
        in R's default
        plot function? Thanks.


    In base graphics, you can have as many axes as you like,
    displaying anything.  Use the axis() function.  See ?axis for the
    arguments that determine placement, ticks, etc.

    I would guess the same flexibility is there in ggplot2, but I
    don't know how to do it.

    Duncan Murdoch