contour_plot and implicit_plot terrible resolution and cutting graph

Hi, I'm using wxMaxima 0.8.5 atop maxima 5.21.1 on Debian Squeeze.

I need to graph the following expression that I have defined as a
function in maxima:

ygs(x,C,d,t) := x * ( ( 1 + 3 * C / x * log( 1/2 + 1/2 * sqrt( 1 + 8 *
x / ( 3 * ( 1 + d^2 ) ) ) ) )^2
+ ( t - 3 * C * d / x * log( 1/2 + 1/2 * sqrt( 1 + 8 * x / ( 3 * ( 1 +
d^2 ) ) ) ) )^2 );

So I'm using both implicit_plot and contour_plot as follows:

implicit_plot( ygs(x,8,d,0) = 100 , [d,-20,20] , [x,0,120] )$

contour_plot( ygs(x,8,d,0) , [d,-20,20] , [x,0,200] )$

As soon as you do this you will notice 3 things:

1. The plot where ygs(x,8,d,0) = 100 is incomplete! it has a gap in the middle
2. There appears to be a mysterious line running parallel to the d-axis
2. The resolution is quite poor

Now for what I'm doing both x and the value of the function, that is
ygs, cannot possibly be negative, but I decided to give this a try:

implicit_plot( ygs(x,8,d,0) = 100 , [d,-20,20] , [x,-200,200] )$

So the mysterious lines running parallel to the axis are explained.
But I don't want these to show in the plot and interfere with the
display of the graph I do want.

Therefore I have the following questions and comments:

1. Is it possible to tell contour/implicit_plot's numerical solver to
disregard anything where x < 0 and/or ygs < 0 ?. I tried using assume
but it seems to be ignored.
2. Is this why there is a gap in the plot?
3. Is there a way to increase the resolution so that the plot traces
don't look so bad? I couldn't find anything in the maxima help.
4. Is this a bug? if so, is this the proper place to report it?
5. For comparison I ran roughly the same commands in Mathematica 8.0.1
for Linux and it had no trouble displaying this curve and it displayed
nothing running parallel to the axis

Thanks in advance for any assistance!

Andres Cimmarusti

PS: I have reproduced the same issue running wxMaxima 11.04 atop
5.24.0 on Debian testing