GrADS: How to plot a wind circulation pattern at a particular atmospheric pressure level with "u" and "v" wind vector components?

The script:

********wind_at_870hPa****for_2007JJAS*****

'reinit'

*'set display color white'

*'c'

'set xlopts 4 8 .2'

'set ylopts 3 6 .2'

'set xlint 6'

'set ylint 5'

*'set grads off'

'set grid off'

'run colors'

*****************

'open file8.nc.ctl'

'set z 18'

*'set mpdset rupres'

*'set cthick 3'

*'set ccolor 1'

*'set mpt 0 15 1 1'

*'set csmooth on'

*'set arrowhead 0.03'

'define uavg=ave(ua,t=101,t=104)'

'define vavg=ave(va,t=101,t=104)'

*'color 1 19 3 -kind rainbow'

*'set clevs  2 4 6 8 10 12 14 16'

*'set ccols 0 48 47 46 45 44 43 42 41'

'define magw=mag(uavg,vavg)'

*'set ccolor 1'

'set gxout shaded'

'd magw'

'run vec.gs skip(uavg,10);vavg -SCL 0.25 15 -P 7.7 9'

'run cbarn'

'draw title Wind at 870hPa for 2007 JJAS'

*'printim output.eps'

'printim output.pdf'

*'printim output.png'

Output image