R ggplot geom_bar Error: Discrete value supplied to continuous scale

The problem is the unnecessary use of cbind inside data.frame. cbind creates a matrix. A matrix must have all values of the same mode (numeric, character, etc.). Since at least one of variables (two in this case) is character mode, cbind coerces Val to character as well. data.frame converts the three character variables to factor. Either way, Val is a discrete (categorical) value rather than numeric, resulting in an error when you use scale_y_continuous.

Change to dfm2 <- data.frame(Group,Statistic,Val) and the error will go away.

You can check the effect of cbind and data.frame on data types as follows:

cbind(Group, Statistic, Val)

     Group Statistic Val   
[1,] "a"   "Mean"    "0.75"
[2,] "b"   "Mean"    "0.83"
...
[7,] "c"   "d"       "0.1" 
[8,] "d"   "d"       "0.3" 

dfm2<-data.frame(cbind(Group,Statistic,Val))
str(dfm2)

'data.frame':   8 obs. of  3 variables:
$ Group    : Factor w/ 4 levels "a","b","c","d": 1 2 3 4 1 2 3 4
$ Statistic: Factor w/ 2 levels "d","Mean": 2 2 2 2 1 1 1 1
$ Val      : Factor w/ 8 levels "0.02","0.1","0.3",..: 6 8 7 5 4 1 2 3

dfm2 <- data.frame(Group,Statistic,Val)
str(dfm2)

'data.frame':   8 obs. of  3 variables:
$ Group    : Factor w/ 4 levels "a","b","c","d": 1 2 3 4 1 2 3 4
$ Statistic: Factor w/ 2 levels "d","Mean": 2 2 2 2 1 1 1 1
$ Val      : num  0.75 0.83 0.79 0.69 0.5 0.02 0.1 0.3

If you don't want data.frame to convert strings to factors, add the argument stringsAsFactors=FALSE.

Try the following.

ggplot(dfm2,aes(x = Group,y = as.numeric(as.character(Val)))) +    
  geom_bar(aes(fill = Statistic),position = 'dodge',stat='identity')+
  scale_y_continuous(limits=c(0, 1))