fig, (ax,ax2)=pl.figure(figsize=(10,5))
ax.plot(x,y1,color='darkgreen',label='test 1')
ax.plot(x,y2,color='lightgreen',label='test 2')
ax.plot(x,y3,color='mediumseagreen',label='test 3')
ax.legend(bbox_to_anchor=(0., 1.02, 1., .102), loc=3,
           ncol=4, mode="expand", borderaxespad=0.0,framealpha=0.0)

ax2=ax.twinx()
ax2.plot(x,y,color='b',marker='.',label='Add to primary legend')