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))

I ran into this same error and solved it by running:

dev.off()

and then running the plot again. I think the graphics device was messed up earlier somehow by exporting some graphics and it didn't get reset. This worked for me and it's simpler than reinstalling ggplot2.

I found this to occur when you mix ggplot charts with plot charts in the same session. Using the 'dev.off' solution suggested by Paul solves the issue.

The solution is to simply reinstall ggplot2. Maybe there is an incompatibility between the R version you are using, and your installed version of ggplot2. Alternatively, something might have gone wrong while installing ggplot2 earlier, causing the issue you see.