ggplot2：从纵横比中排除图例

I use ggplot2 and knitr发布带有右侧图例的散点图。图例包含在纵横比中，因此破坏了绘图的“方形”，如图所示默认主题 https://github.com/hadley/ggplot2/wiki/themes。当图例文本变得比“a”和“b”长一点时，图形就会变成“长矩形”而不是“正方形”。

我想保持图表“平方”，因此想从我的纵横比中排除图例ggplot2图表。我的.Rprofile有以下信息强制ggplot2生成带有更大文本和轴标题周围更多空间的低色图表：

theme_set(theme_bw(16))
theme_update(
  axis.title.y = element_text(angle = 90, vjust = -.25),
  axis.title.x = element_text(vjust = -1),
  plot.margin = unit(c(1,1,1,1), "cm")
)

我可以在此处添加任何内容以从宽高比中排除图例吗？操纵与coord_equal and coord_fixed到目前为止都失败了，同样fig.width and fig.height选项。感谢您的帮助！

Edit:删除了工作示例，下面用完整的示例代码回答了问题（很抱歉延迟批准答案）。

Setting coord_fixed()应该做的伎俩：

library(ggplot2)
library(gridExtra)  ## for grid.arrange()

## Create some data with one longer label
cars <- transform(mtcars, 
           cyl = factor(cyl, labels=c("4","6","8 is big")))


## Compute ratio needed to make the figure square
## (For a taller narrow plot, multiply ratio by number > 1; 
##  for a squatter plot, multiply it by number in the interval (0,1))    
ratio <- with(cars, diff(range(mpg))/diff(range(wt)))

## Make plots with and without a legend
a <- ggplot(cars, aes(mpg, wt)) + 
     geom_point() + coord_fixed(ratio=ratio)

b <- ggplot(cars, aes(mpg, wt, color=cyl)) + 
     geom_point() + coord_fixed(ratio=ratio)

## Plot them to confirm that coord_fixed does fix the aspect ratio
grid.arrange(a,b, ncol=2)