Adding Year-Month-Day to X axis

Thanks. Regarding 
axis(1,at=x_yyyymmdd,labels=format(x_yyyymmdd,"%Y-%m-%d"))

How do I get the text for YYYY-MM-DD to be drawn vertically, instead of horizontally?
Greg

On May 6, 2018, at 11:54 PM, Jim Lemon <drjimlemon at gmail.com> wrote:

axis(1,at=x_yyyymmdd,labels=format(x_yyyymmdd,"%Y-%m-%d"))