Question: Bug in Maple 9 plot?

Has anyone encountered a bug (or bugs) in Maple 9's plot functionality? I am using Maple 9 (probably need to upgrade!) and getting odd plots.

For example, I have a procedure called T_opt(x,y,z) that fines the optimal T given parameters (x,y,z), and I have checked the values for X_opt directly. For instance, below we see that T_opt is decreasing in x:

> T_opt(0.2,0,0.5);
0.917116057083333324
> T_opt(0.3,0,0.5);
0.854603175416666638
> T_opt(0.4,0,0.5);
0.812654453124999954

But when I plot T_opt as follows:

> plot([T_opt(x,0,0.5),T_opt(0.4,0,0.5)],x=0..0.5);

the plot shows T_opt increasing in x! Basically, totally wrong values for T_opt are shown in the plot, i.e. not the values above!!

See attached plot.

Note, I have also seen some similar bugginess when using plot3d. Is anyone aware of this problem? Or am I plotting T_opt incorrectly somehow? Thanks.