how to set xdate format from " May 6 2008" to "2008-05-06"?

```
using PyPlot;
clf();
close();
fig =figure();
ax = fig[:add_subplot](111)
x = [Date(now()) + Dates.Day(i) for i in collect(1:100)]
y = rand(100)
ax[:plot](x,y, color="red", linewidth=2.0, linestyle="-")

```

but i get x autofmt_xdate ,format such as " May 6 2008"
how to set xdate format as "2008-05-06"?
thanks.