Because the ...
is other aesthetics passed for each layer, i.e. you are not setting alpha
you are mapping it to some value which is then the same for all values. You can tell by how you also get a legend entry for alpha
on the plot. There are two solutions:
1) Use the I
to indicate this is a set aesthetic;
qplot(a,b,data=dt, geom="point", alpha=I(0.1) )
2) Instead use a ggplot
and set the aesthetic in the geom
...
ggplot( dt , aes( a , b ) )+
geom_point( alpha = 0.1 )
Both calls produce the same result.