Hello,

I am trying to use vspan to highlight specific periods in a plot with dates as x axis. It seems that vspan changes the y axis units to dates also (which is wrong).

MWE :

```
# Show vspan problem with date abscissaes
using Plots
using Dates
gr()
#ENV["GKSwstype"]="nul" # from batch / no X11
#ext=".svg"
ext=".png"
sf=true
y=[1.0*i*i for i in 1:10]
@. y= -y + 5.0
# x=[i+0.0 for i in 1:10]
x=[Date(2019,11,i) for i in 1:10]
rx=[x[3],x[5]]
ry=[y[3],y[5]]
p=plot(x,y, label="y", title="Vspan problem with dates")
if sf; savefig(p,"p-plot"*ext); end
p=vspan!(p,rx,label="",alpha=0.2)
if sf; savefig(p,"p-vspan"*ext); end
```

Here is the (incorrect ?) result

What can I do to go around this ?

Thanks in advance,