Gap between bars in bar graph

Below is a worked example:

The data set I used is auto.dta. The commands I used in a do-file is:

sysuse auto, clear
graph bar (mean) price turn mpg weight, over(rep78)
set obs 75
replace rep78=2.5 in 75
graph bar (mean) price turn mpg weight, over(rep78,  relabel( 3 " "  ))

If you would like to know more on Stata graphs, you will find the book A Visual Guide to Stata Graphics, 3rd Edition very useful.