markers not drawn if x world coordinate reversed

example:

```
import vcs
import numpy
import MV2
import cdms2
x=vcs.init()

data = MV2.array([4,5,6,7,1,3,7,9,])+230.

p = cdms2.createAxis([2,5,100,200,500,800,850,1000])

data.setAxis(0,p)

data.id="jim"

gm=x.create1d()

gm.linewidth=0
gm.datawc_x1=1000
gm.datawc_x2=0

gm.markersize=30

x.plot(data,gm)
raw_input("Press enter")
```
