[R] gplot problems with faceting

[BOISSON, Pascal]

Dear R-Listers,

I am very confused with what seems to be a misuse of the faceting options with gplot function and I hope you might help me on this.

z contains various simulation results from simulations with different set of parameters.
I melt my data to have the following data.frame structure : 

> str(z)
'data.frame':   12383 obs. of  5 variables:
 $ vID      : num  1 2 3 4 5 6 7 8 9 10 ...
 $ Var      : Factor w/ 61 levels ".t",".ASU_1.Biofilm_C",..: 1 1 1 1 1 1 
 $ Var.Value: num  317 318 319 320 319 ...
 $ Par      : Factor w/ 7 levels ".Biostyr0d.t_K",..: 1 1 1 1 1 1 1 1 
 $ Par.Value: num  5 5 5 5 5 5 5 5 5 5 ...

I would like to plot for each couple (Parameter(i), Variable(j)) the plot Variable(j).value = f(Parameter(i).Value.
I would like to do it step wise and have one set of graphs per Variable.
Then I subset z based on a single variable name eg ".ASU_1.Biofilm_C"

Then I try the following, but I get an error message :

> qp<- qplot(Par.Value, Var.Value, data = z[z$Var==v,], ylab=v, geom=c("point","smooth"), method="lm")
> qp<- qp + facet_wrap( facets= Par~ ., scales = "free_x", ncol=length(vPar))
> qp
Erreur dans `[.data.frame`(plot$data, , setdiff(cond, names(df)), drop = FALSE) : 
  colonnes non définies sélectionnées

I can have this working by modifying the facets arguments to "Par~Var", and it does what I want, 
But it is not satisfying, and I am confused with this error message.
The same error message happens when I use the full data frame. 
Or when I try other mappings like colors = Par

Any idea of what I am doing wrong?

Best regards
Pascal Boisson
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