typo in knitr label
better distinction between subset and where
added better than SQL paragraph
added lm and grouped quantiles
combine arg no longer used
more readable with negative sign added later
handle 0 row results
handle 0-row data frames
handle NULL edge cases
don't go too low with N ever
switch to using Hmisc instead of hydroPSO for weighted quantiles
use counts from plyr, skips 0 counts
make this work independent of number of grouping cols
renamed
swap hydroPSO dep out for Hmisc
catch the null case
add column selection as vectors
even safer cbind for 0 col data frames
renaming quantile.cols
factored out env copy