单位库简化了单位的使用。据我所知，使用单位进行绘图可以与基本绘图一起使用，但不能与ggplot一起使用。有什么建议？

library(units)

# Take cars data frame: stopping dist (ft) vs speed (mph)
plot(cars)

# units + base plot
Distance = set_units(cars$dist, ft)
Speed = set_units(cars$speed, mph)
plot(x=Speed, y=Distance) #Clean

# units + ggplot
library(ggplot2)
df = cars
df$Disance = set_units(df$dist, ft)
df$Speed = set_units(df$speed, mph)

qplot(x=Speed, y=Distance, data=df)
# Error in Ops.units(x, range[1]) : 
#   both operands of the expression should be "units" objects