I had faced this issue in the past. I did bellow changes to resolve the issue.
Firstly you need to Change sample (your built-in function) to Sample (your variable)
ggplot(data=data1, aes(x=Sample, y=value, fill=variable)) +
geom_bar(width=1) + scale_y_continuous(expand = c(0,0)) + opts(axis.text.x=theme_text(angle=90))