Broken pipe - Lisp debugger



I have installed Maxima, WxMaxima and Gnuplot on my Mac computer (OsX).
I am able to type in general equations/functions into Maxima and/or WxMaxima
without any trouble.  However, when I try to plot graphs, I get the
following message:

*plot2d: expression evaluates to non-numeric value somewhere in plotting
range.*
*plot2d: expression evaluates to non-numeric value somewhere in plotting
range.*
*Maxima encountered a Lisp error:*
* Couldn't write to #<SB-SYS:FD-STREAM for "descriptor 7" {13269D91}>:*
*   Broken pipe*
*Automatically continuing.*
*To reenable the Lisp debugger set *debugger-hook* to nil.*



I would like some advice on what to do to fix this problem.

Gnuplot seems to work ok (and produce graphs) when I type commands straight
into the gnuplot terminal.  Also, I downloaded Maxima onto a PC and the
graphs automatically appeared when I entered the same graphing command into
WxMaxima on the PC.  It seems the Mac is the problem.  I suspect it has
something to do with Maxima and Gnuplot not properly connecting, but I don't
know how to remedy this.

In case you're wondering - I have downloaded aqua and I have X11.

Thank you.