Filled step plots in matplotlib

It's not possible to do a simple filled step plot in matplotlib using default
commands. Workaround:

def steppify(arr,isX=False,interval=0):
    """
    Converts an array to double-length for step plotting
    """
    if isX and interval==0:
        interval = abs(arr[1]-arr[0]) / 2.0
        newarr = array(zip(arr-interval,arr+interval)).ravel()
        return newarr

plot(xx,yy,linestyle='steps-mid',color='b',linewidth=1.5)
fill_between(steppify(xx[x1:x2],isX=True),
    steppify(yy[x1:x2])*0,
    steppify(yy[x1:x2]),
    facecolor='b',alpha=0.2)