画堆积条形图，横轴是日期day，取值有几百个，画出来显示的横轴标签黑成一片，如图，看不清楚。
如何才能显示某几个关键日期，或者以每周/每月的间隔显示日期，请大家帮忙看看，非常感谢！

指令和部分数据如下：
gen a=.
recode a .=1 if classification=="a"  
gen b=.
recode b .=1 if classification=="b"
gen c=.
recode c .=1 if classification=="c"
gen day2=date(day,"YMD")
format %td day2
gen day2_text=string(day2,"%td")
egen day2_pos=group(day2)
labmask day2_pos, values(day2_text)
graph bar (count) a b c, over(day2_pos,gap(0)) stack

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