[Matplotlib-users] open ascii grid data and plot

I have a txt file (with an associated prj file) containing gridded weather
data.
Firstly how can I open this file and convert it to a numpy array?
and then how to plot in matplotlib, paticularly how to use the lat, lon and
nrows,ncols.
ncols=886
nrows=691
longitude west=111.975, east=156.275
latitude north=-9.975, south=-44.525
cell size=0.05

My attempt below:
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onefile=open("E:/test_in/r19000117.txt", 'r')

map = Basemap(projection='merc',llcrnrlat=-40,urcrnrlat=-33,
              llcrnrlon=139.0,urcrnrlon=151.0,lat_ts=0,resolution='i')
map.drawcoastlines()
map.drawstates()
xi=N.linspace(111.975, 156.275, 886)
yi=N.linspace(-44.525, -9.975, 691)
x,y=map(*N.meshgrid(xi,yi))
plt.title('rainfall')
CS = map.contourf(x,y, onefile, cmap=plt.cm.jet)
l,b,w,h =0.1,0.1,0.8,0.8
cax = plt.axes([l+w+0.025, b, 0.025, h])
plt.colorbar(CS,cax=cax, drawedges=True)
plt.savefig((os.path.join(OutputFolder, 'rainfall.png')))
plt.show()
plt.close()