我能改变x轴在ggplot2中与y轴相交的位置吗?

I'm plotting some index data as a bar chart. I'd like to emphasise the "above index" and "below index"-ness of the numbers by forcing the x-axis to cross at 100 (such that a value of 80 would appear as a -20 bar.)I'm plotting some index data as a bar chart. I'