如何使用 ggplot 创建并排条形图（针对多个系列）？

我有两组数据（3 列：x=分类、y=数值、l=位置），我想创建一个条形图，其中 x 轴上有类别，并且对于类别的每个值，有两个垂直条，颜色不同，每个位置的 y 值。默认情况下，Excel/OpenOffice 会生成这种图表。

I tried

qplot (x,y,data=mydata,col=location, geom="histogram")

但它会产生堆叠的条形，而不是并排的。然后我查看了 ggplot2 文档，没有找到任何我可以使用的其他几何对象（完整列表见下文）。

ggplot2 不可能吗？

提前致谢。

Name Description
abline - Line, specified by slope and intercept
area - Area plots
bar - Bars, rectangles with bases on y-axis
blank - Blank, draws nothing
boxplot - Box-and-whisker plot
contour - Display contours of a 3d surface in 2d
crossbar - Hollow bar with middle indicated by horizontal line
density - Display a smooth density estimate
density_2d - Contours from a 2d density estimate
errorbar - Error bars
histogram - Histogram
hline - Line, horizontal
interval - Base for all interval (range) geoms
jitter - Points, jittered to reduce overplotting
line - Connect observations, in order of x value
linerange - An interval represented by a vertical line
path - Connect observations, in original order
point - Points, as for a scatterplot
pointrange - An interval represented by a vertical line, with a point
in the middle
polygon - Polygon, a filled path
quantile - Add quantile lines from a quantile regression
ribbon - Ribbons, y range with continuous x values
rug - Marginal rug plots
segment - Single line segments
smooth - Add a smoothed condition mean
step - Connect observations by stairs
text - Textual annotations
tile - Tile plot as densely as possible, assuming that every tile is the same size
vline - Line, vertical

有一个position默认为的参数stack这里。使用：

qplot (x,y,data=mydata,col=location, geom="bar", position="dodge") 

说明书上有，搜索“dodge”就可以了。另外，如果y值给出了条形的高度。