x.close() brokn if more than one plot on

```
import vcs
import os,cdms2
f=cdms2.open(os.path.join(sys.prefix,"sample_data","clt.nc"))
s=f("clt")
x=vcs.init()
y=vcs.init()
x.plot(s)
y.plot(s)
x.close() # only clears
y.close() # only clears
```
