Persistent invalid graphics state error when using ggplot2

I believe my dataframe is okay and my code is okay. In fact, I have eliminated parts of the dataframe and most of the graphing code to make things as basic as possible. But still, I get:

Error in .Call.graphics(C_palette2, .Call(C_palette2, NULL)) : 

  invalid graphics state

What is wrong here? Here is the data:

 date   trt var val

1/8/2008    cc  sw5 0.2684138

1/8/2008    cc  sw15    0.2897586

1/8/2008    cc  sw5 0.2822414

2/8/2008    cc  sw5 0.2494583

2/8/2008    cc  sw5 0.2692917

2/8/2008    cc  sw15    0.2619167

2/8/2008    cc  sw5 0.204375

3/8/2008    cc  sw5 0.2430625

3/8/2008    cc  sw5 0.2654375

3/8/2008    cc  sw5 0.2509583

3/8/2008    cc  sw5 0.2055625

1/8/2008    ccw sw15    0.2212414

1/8/2008    ccw sw5 0.3613448

1/8/2008    ccw sw5 0.2607586

2/8/2008    ccw sw5 0.2087917

2/8/2008    ccw sw15    0.3390417

2/8/2008    ccw sw5 0.2436458

2/8/2008    ccw sw5 0.290875

3/8/2008    ccw sw5 0.20175

3/8/2008    ccw sw15    0.328875

3/8/2008    ccw sw5 0.2328958

3/8/2008    ccw sw5 0.2868958

When I work with this data, I specify dates like this:

df<-df[order(as.Date(df$date,format="%d/%m/%Y")),,drop=FALSE]

and here I want to make a scatterplot:

ggplot(data = df,aes(x = date,y = val)) + 

 geom_point(aes(group = trt))